Monday, May 31, 2010

Barrel Making: Pattern Welded or Damascus Barrels

In our last post about early barrel making techniques in Europe, one of the techniques we studied was the making of the Canon a Ruban type barrels in France. On reading that article and viewing the picture of that barrel, the reader cannot but note the beautiful striping pattern on the barrel showing the grain of iron. Now we will study another method of manufacturing barrels, that originated not in Europe, but in India!

First we start with something called "Damascus Steel". What is popularly known in Europe as "Damascus Steel" was really a type of steel called wootz steel that originated in Southern India around 300 BC. The crusaders originally encountered swords made of this steel in Damascus, Syria and that's how it got its name. These steels were noted for their sharpness and toughness. One of the characteristics of swords made wootz steel is a banding pattern on the blade. Thus, any sword with the characteristic banding pattern was considered to be extremely high-quality.

While the wootz steel owed its qualities to certain impurities found in the ore from a specific mine in India, another way was found to reproduce the beautiful wavy patterns on a blade.

This technique was called "Pattern Welding" and was known to several cultures indeed. The Japanese used it to manufacture their swords since 1100 AD and the Vikings and Celts were using it around 600 AD, as were the ancient Germans. The idea was to use bars of two or more types of steel (0r iron and steel), one having less carbon content than the other and forge them together into a single bar, by heating, twisting and hammering as needed and then fold the bar onto itself and hammer forge it again and repeat the process of heating, twisting, folding and hammering multiple times, resulting in a bar with layers of steel of different types. Such a bar is called a pattern welded or laminated steel bar. The multiple repeated processes of twisting, folding and hammering causes the resulting steel to be purified of impurities and form a tougher steel. The resulting steel has wavy lines and patterns visible due to the difference in chemical composition between the different bars used. An example of steel produced with this method is shown below. Note the beautiful figurations on the blade, which is characteristic of pattern welded steel:

By 1570, pattern welding was used in the manufacture of gun-barrels in India, according to the Ain-i-Akbari written by Abul Fazl, the court historian of the Mughal emperor Akbar of India. From volume 1, we have the following chapter:

"These are in particular favour with His Majesty, who stands unrivalled in their manufacture, and as a marksman. Matchlocks are now made so strong, that they do not burst, though let off when filled to the top. Formerly they could not fill them to more than a quarter. Besides, they made them with the hammer and the anvil by flattening pieces of iron, and joining the flattened edges of both sides. Some left them, from foresight, on one edge open but numerous accidents were the result, especially in the former kind, His Majesty has invented an excellent method of construction. They flatten iron, and twist it round obliquely in form of a roll, so that the folds get longer at every twist then they join the folds, not edge to edge, but so as to allow them to lie one over the other, and heat them gradually in the fire. They also take cylindrical pieces of iron, and pierce them when hot with an iron pin. Three or four of such pieces make one gun or, in the case of smaller ones, two. Guns are often made of a length of two yards those of a smaller kind are one and a quarter yards long, and go by the name of bamdnak. The gunstocks are differently made. From the practical knowledge of His Majesty, guns are now made in such a manner that they can be fired off, without a match, by a slight movement of the cock."

Around the early 1600s, the technique had spread to the Ottoman Empire and later to Hungary and Spain by the 1650s. The defeat of the Turks in the Siege of Vienna in 1683 yielded thousands of captured pattern welded barrels for examination, and this event accelerated the manufacture of pattern welded barrels in Europe. By 1700, the Belgians were producing pattern welded barrels in Liege, and in the early 1800s, the technique was used in England to produce high quality sporting barrels.

Even though making steel through pattern welding is a vastly different process than producing wootz steel, they both have similar wavy watering patterns in the final product. However, since wootz steel was becoming rarer, due to the fact that the mine in India where the special ore was mined from was running out of ore, it wasn't used to make gun barrels very much. William Greener in his Gunnery in 1858: Being a Treatise on Rifles, Cannon and Sporting Arms writes that on examination of barrels made by wootz steel workers, most were actually were made of commonest iron with a very thin plate of wootz steel around them, indicating that the wootz steel ore was becoming very valuable. In fact, when anyone refers to "damascus barrels", they are almost certainly referring to barrels made by the pattern welding method, not barrels made out of wootz steel.

We will refer to some details about the technique of pattern welding used in India, written by Lord Egerton of Tatton, from his book Indian and Oriental Arms and Armor, published in 1896. This section is reproduced from an article from that book, found on

It is said that the Persians distinguish by ten different names the varieties of watering. One of the most prized and rare is that which takes its name from the grains of yellow sand. There are, however, four main patterns generally recognized:

1. " Kirk narduban," meaning the forty steps or rungs of the ladder, in allusion to the transverse markings of fine grey or black watering. The idea is also expressed in an inscription on one of the blades, that the undulations of the steel resemble a net across running water.

2. " Qara khorasan," nearly black, with fine undulations proceeding like water either from the point to the hilt, or the reverse way.

3. Qara Taban, " brilliant black," with larger watering and more grey in tone.

4. Sham, or simple Damascus, including all other varieties. On the introduction of the use of firearms, the methods long and perhaps exclusively known to the Asiatics, of manufacturing sword-blades of peculiar excellence, was transferred with some modification to that of gun-barrels, and are still in use.

In addition to those, Sir J.I. Burnes mentions " Akbaree," in which the pattern ran like a skein of silk the whole length of the blade, and "Beguraee," where it waved like a watered silk.

In Persia, Kabul, the Punjab, and Hind the same general principles prevail, but the matchlocks of the last are held deservedly in the highest estimation.

In some parts of India the workmen prefer for the material of their barrels the iron of old sugar boilers, but they use in Kashmir the iron of Bajaur (in the country of the Yusufzai) as it comes from the smelting furnace, after receiving a few blows whilst hot, which condense it into a rude kind of pig, the weight of which varies from five to eight seers (10 to 16 Ibs.), and which sells
as high as 4d. a pound. The first process consists in cutting the pig when heated into narrow strips with a cold chisel, and in this operation the iron loses one-fourth of its gross weight. Each of these strips separately is brought to welding heat, and worked smartly under the hammers of two men on a block of limestone as an anvil. When the slag is expelled, each strip is drawn out by the hammer into a strap about 2 feet long and 11/5 inch broad, and 1/5th inch thick. One of these straps has its ends so brought together as to enable it to include about 20 other short straps cut up for the purpose, some being placed on their edge, and others wedged in between the lengths, sо as to form a compact mass. It is then put into the fire and lightly heated, receiving a few blows upon both faces as well as upon the edges.

It is next smeared over with a paste of clay and water, and when dried it is exposed first to a light welding heat, and after a slight hammering to a stronger heat, when it is vigorously and quickly beaten into four-sided bars about a foot long, and a finger's thickness. These are again heated, separated, and drawn out into square rods about J-inch broad on each face. These are then twisted from right to left, while the part which is to be twisted is heated to a red heat nearly verging upon white. This process is repeated by heating two or three inches at a time, and then cooling it with cold water, till the whole rod is converted into a fine screw, which is made as even as possible.

To make an Iran barrel six or eight rods are required. When eight are employed, four of them have the twist from right to left, and four from left to right. Every rod after having been slightly heated is lightly hammered on its two opposite sides equally, so that two sides have the threads beaten down, and the two others have the threads standing, and retaining their original roundness. Each rod is now made up of lengths of the same direction of twist, and is laid parallel to the other, so that rods of opposite twist are in alternate succession.

The steel having been formed into bars is now ready for manufacture into gun-barrels.

The extremities of the bars are welded together, and the baud or skelp is now ready for being formed into a hollow cylinder through being twisted in a spiral line upon itself, which is begun at the breech or thicker end, and continued to the muzzle. When the twisting is so far completed that the edges of all the twists stand even, and the cylinder is nearly equal, it is coated with a thin paste of clay and water, and is then ready for being welded.

A welding heat is first taken in the middle of the cylinder, and the edges of the twists are brought together by the breech being struck down upon the stone anvil perpendicularly for the purpose of jumping up the edges. The welding is constantly repeated, so that the twist, which was jumped up, is successively hammered when the heat is well on, till the barrel has been
welded up to the muzzle.

This process is then repeated, commencing from the middle to the breech, and afterwards from the middle to the muzzle, during which an iron rod is introduced at each end and used as a mandrel. A third heat nearly red is now taken at the whole surface of the barrel, which is then made regular and level by smartly hammering it. The barrel is then fixed horizontally through
a hole in an upright post and bored, after which its surface is filed, polished, and prepared for bringing out the damasked lines. " Jauhar " is brought out through biting the whole surface with " kasis," a sulphate of iron.

The barrel is completely freed from grease or oil by being well rubbed with dry ashes and a clean rag. About three drachms of sulphate of iron in powder is mixed with as much water as is sufficient to bring it to the consistence of thick paste which is smeared equally over the whole surface of the barrel, the muzzle and breech being at the same time carefully plugged. About two hours afterwards, when the metal has assumed a blackish colour, the coating is rubbed off, and the barrel cleaned as before. Barrels are called " pechdar" when plain or simply twisted, "jnulmrdar tvheu dunmsked" when damasked. For the latter the rods are disposed according to the kind of brilliant or damasked lines to be produced, called either from the country as "Iran" or Persian, or from the figure, as " pigeon's eye," " lover's knot," " chain".

The barrel is then smeared with a preparation composed of the same quantity of sulphate of iron and four ounces of water, and is hung up in the well. Every gunsmith has, in the floor of his shop, a well about two yards deep, the bottom of which is covered with a layer of fresh horse-dung half a yard thick. Suspended by a string from the cross stick at the mouth of the well, the barrel which has been covered with the mixture as before is taken out every morning and cleaned with dry ashes and cloth, and hung up for 24 hours with a coating of the solution. This process is continued for 20 days or a month till prominent lines are formed on the surface of the barrel, separated from each other more or less by other depressed lines or grooves; the former will be found to have the same direction as that of the thread of the screw in the twisted rods. The prominent lines when rubbed are bright and of a colour somewhat approaching silver, while the depressed lines are dark and form the pattern.

The " zanjir " or chain damask consists in the introduction of a band of prominent and brilliant lines disposed like the links of a chain between parallel plain lines of damask. The processes are the same as before described in cutting up the " pig," and in reducing the strips into straps, but the " pie " or " ghilaf " contains only eight lengths, which when welded is drawn out into straps 1/2 inch broad and 1/8 inch thick. One of these straps being heated is bent backwards and forwards upon itself in eight continued loops, each an inch long, and is then worked up into straps 1/3 inch broad, and 1/16 inch thick.

Three of this kind of strap are required in this pattern, one for the chain and two for the lines. The face of the iron anvil has a perpendicular hollow about one-quarter of an inch deep, and about one-third of an inch across. One end of the strap is laid while cold across this groove, and driven down into it by a small chisel and hammer, by which the strap receives a bend or angle. Its opposite face is then placed across the die near the acute elbow made by the chisel, and is in like manner wedged into it, after which the operation is reversed until the whole band is converted into a frill of loops. This frill is then heated, and the operator holding one end with a small pair of tongs brings two pairs of loops together leaving the ends open. This is continued till the frill is much reduced in length through the loops of the strap standing at right angles to its general direction. Different lengths of frill are welded together, so as to form a ribbon six spans long, placed in contact with two plain straps set on edge, and four rods, two on each side
twisting alternately, from left to right, and the reverse. The general band of these seven straps is then treated as that for the "Irani" damask.

The chain damask is in general preferred to all other varieties, excepting the silver twist. The Kashmiris still make blades for daggers in the same way, as one which was made for the author at Srinagar to fit an Indian jade handle is damasked, and Moorcroft relates that they made sword blades for him to order, though they did not usually manufacture them. It is said that "jauhar" is imitated in Hindustan by lines being traced in a coating of wax laid over the metal, and the barrel being exposed to the action of sulphate of iron.

We will now look at some pictures of barrels produced in India using such techniques. The first is an Indian matchlock with a herringbone pattern barrel:

(click image to enlarge)

The next is an Indian matchlock from the late 1700s with a beautiful Crolle-twist wave pattern:

(click image to enlarge)

The next is an 18th century Indian musket with a laminated steel barrel:

(click image to enlarge)

Note that while the barrel making technique in India was quite advanced in the 1570s, they were still using ancient matchlock firing mechanisms by the time Lord Egerton wrote his book in the late 1800s. In the next post, we will look at the manufacture of pattern welded barrels in Europe.

Saturday, May 29, 2010

Barrel Making: Early Barrel Making in Europe

In our last post, we talked a bit about the gunmaker guilds and how they affected gun development in Europe. We also talked about how the early barrel makers were blacksmiths. We will visit some of their history in this post.

The blacksmiths of Italy and Spain that made the early gunbarrels usually used iron from old horse-shoe nails. There was no specific reason to prefer iron from horse-shoe nails, merely that a majority of the early barrel-makers just happened to be the same blacksmiths that dealt with making horse shoes and fitting them. In modern times, a person specializing in making and fitting horses shoes is called a farrier, but in the middle ages, the job of the farrier and the blacksmith were practically synonymous and the terms were used interchangeably.

The method used in Spain was to weld a bunch of nails into a strip of iron and then bend this into a cylinder about 5-6 inches in length. The strip is curled around itself twice so that the walls are double throughout the cylinder, for extra strength. To form a barrel of a certain length, multiple cylinders are selected and arranged end to end and welded together to form the barrel. The advantages of this method are that the metal is forged in smaller sections, so it is better wrought and purified. Also, if one of the cylinders has cracks or weld defects during its forging process, it can simply be discarded and another one substituted in place, when joining all the cylinders together to make a new barrel.

When this method was used, there were a lot of metal loss during the forging process. About 20 kg. of nails were used to make a barrel that weighed just 2.5-3 kg., but the resulting barrel was light and strong. Martinez del Espinar, the personal gun-bearer of King Philip IV of Spain, mentions that a quality gun barrel 1 metre long that is forged with this method should weigh just 2 kg. Making these barrels was labor intensive and expensive and hence this technique was used to manufacture barrels for the finest sporting guns of that period.

Another method of barrel manufacturing also became popular in France in the late 1700s. Barrels produced by this method were called Canon a Ruban or "Ribbon barrel". The method of manufacture, as stated by Marolles is as follows:

First, the smith starts off with a sheet of iron that is much thinner than the required barrel. This sheet is rolled into a thin tube that is the length of the barrel and slightly smaller than the required barrel diameter. This tube is called a chemise. Then a thicker strip of iron about an inch broad and chamfered to a point on either edge is heated a few inches at a time and wound around the chemise. This strip is called a ruban (i.e.) a "ribbon". To roll it around the chemise, they use a pair of tongs where one beak is short and flat and the other is rounded and long. The long beak is used to turn and press the strip on to the chemise. Five feet of ruban is used to make one foot of barrel. Since it is hard to make a barrel from a single ruban, the smith often made three of these separately, each one foot long, and then welded them together into a single three-foot long barrel. Then the whole barrel is placed in the furnace to heat it and forge it as a single barrel. Then the barrel is sent to the boring shop, where the chemise (the lining) is mostly removed using a boring bit, leaving behind the ruban forming the barrel.

This method produced a barrel of superior strength, as the welds were transverse to the barrel and could better resist the force of explosion. An example of a double barrel muzzle-loading weapon produced with this method is shown below:

(click image to enlarge)
Note that a part of the "CANON A RUBAN" inscription is visible, engraved between the two barrels, showing that these barrels was made using the above method. Also note the striping on the barrel showing the grain of the ribbon twisted around the chemise. The barrels have a beautiful appearance with figured patterns and have also been browned to protect them from rusting. The process of "browning" will be described later.

Another method that became popular in the early 1800s in Birmingham, was to roll a thick barrel out of a short strip of iron wound round a mandrel. The mandrel was then removed and this barrel would be passed between rollers with tapered grooves to lengthen the barrel, the edges being welded as the barrel passed through the rolls. This method of manufacture was severely opposed by the welding workers associations, as they saw it as a threat to their livelihood. Even though there were several riots, this method was used by several manufacturers because of its suitability for mass production of musket barrels. Muskets made using this method were often low-quality and many were exported to Africa as part of the slave-trade.

Friday, May 28, 2010

Barrel Making: Early Gun Making in Europe

In the last post, we studied the early history of barrel making and different techniques that were used to make early gun barrels. In Europe, the first barrels were made in Italy, probably by smiths in Perugia. At this time, medieval Europe was beginning to see the rise of guilds which were associations of workers of a particular trade. People who were not guild members were not allowed to practice the profession of the guild in question. There were guilds for various professions such as tailors, wine makers, weavers, blacksmiths, carpenters etc. and each guild would guard the secrets of their profession jealously. Accordingly, it was the blacksmiths guilds that had a monopoly of the early gun trade in continental Europe. The centers of gun production were either state owned arsenals such as St. Etienne in France, Brescia in Italy, London in England etc., or in areas where smith guilds were concentrated (in particular, people who made iron nails) such as Liege in Belgium, Suhl in Germany, Bilboa and Eibar in Spain etc. After the early muskets were made, blacksmiths often had to work in collaboration with the carpenters guilds to make the wooden stocks that the barrels were mounted to. This led to craftsmen in some regions forming their own unique "gunmakers guild" which was then regarded as a separate profession from the normal blacksmith and carpenter guilds. The gunsmiths of Suhl formed their guild in 1463.

England was relatively late in the gun-making game and at the time when Henry VIII ascended the throne of England in 1509, there were very few guns in England's arsenal and there was only one expert cannon maker in all of England who knew how to cast guns. Henry VIII quickly rectified the gun shortage situation by importing every gun he could buy from continental Europe and built up a sizeable arsenal very quickly. In fact, by 1513, just before a war with France, Henry placed a large order of guns with the kingdom of Venice and caused the outraged Ambassador of Venice to report back to the Doge that he already had "enough cannon to conquer hell!"

Due to the monopoly of the local guilds throughout continental Europe, many who had learned the profession could no longer practice it if they moved from one town to another, since they were not members of the gunmakers guild of the new town. Luckily for England, the guilds did not have as much power there and so when Henry VIII invited skilled gunmakers to settle in England and carry their trade there, many were only too glad to accept. Arcanus de Arcanis from Italy, Peter van Collen from Belgium, Bawde from France, Cornelius Johnson from Holland and several other skilled gunmakers of that era all made their way to London. By 1545, Henry VIII had plenty of people in his service who knew how to use, repair and make arquebuses. Along with setting them up around the Tower of London (where England's Royal Arsenal was situated), Henry requested that they train local Englishmen in their trade as well. These gunmakers were the start of the gun-making industry in England and for the next few hundred years, the industry was concentrated around the neighborhood of the Tower of London. By the time Henry's daughter, Queen Elizabeth I, ascended the throne, there was thirty-seven gunmakers plying their trade and a Dutchman named Hendricke was the most famous gunsmith in the 1590s. However, King James repealed an act of Queen Mary and granted the monopoly of gunmaking to one Edmund Nicholson and the industry declined to the point that, by 1607, there were only five gunmakers left and they petitioned Parliament to abolish the monopoly so that the "mysteries of gunmaking could be retained." Their grievance was addressed, but it wasn't until 1637 that the London Gunmakers Company was established and this guild later began to dominate England's firearm industry to the detriment of other centers of manufacturing. In fact, there were several legal battles between the members of the London Gunmakers Company and the Birmingham Gunmakers Company in the years to come.

Barrel Making: Early History

We've just finished studying rifling technology in the last 14 posts. Now it might be a good idea to study barrel-making, i.e. the part that the rifling is cut into.

Let's start with the basics... why is a "barrel" called that anyway? To answer that, we have to go to the early days of making barrels. Back in the 1300s and 1400s, metallurgy was not as advanced as it is today, and neither were machining techniques. It was too time consuming to try and drill a hole through a solid cylinder of iron or bronze. So they had to use one or two different techniques to make barrels.

The first inventors of gunpowder were the Chinese and they used bamboo plants as their barrels. Bamboo tree trunks have naturally cylindrical hollow tubes and were plentiful in China as well, so it was natural to use these for small hand-cannons. The main problem with bamboo is that it can't take a lot of pressure, so the early hand-cannons could not fire powerful charges. Another alternative was to wrap the bamboo tubes tightly with rope to strengthen them.

The next technique to be used was casting. One of the oldest types of casting is called Green sand molding. First, a model (called a "pattern") of the barrel tube is made using an easily machined material such as wax or clay. Then the pattern is placed in hollow box frames (called "flasks") with no top or bottom. A couple of cylinders (called the "runner" and riser") are also placed in the box, touching the pattern. Wet green sand is poured into the flask and packed tightly around the pattern, the runner and the riser. After this, the runner, riser and pattern are removed, leaving hollows in the sand. Molten metal is then poured through the hole left by the runner tube and flows into the cavity left behind by the pattern and climbs up the riser. The metal is left to cool and harden and then the sand mold is broken to reveal the hardened casting. In another type of casting called "investment casting" or "lost wax casting", the pattern is not removed, but when molten metal is poured into the runner tube, it melts the wax and pushes it out via the riser tube. Incidentally, the "lost wax" casting method traces its history to India, centuries before guns were invented. Ancient Indians used this technique to make metal figurines and intricate jewelry. The Chinese were the first to employ casting techniques to make metal gun barrels and Arabs and Europeans also used this technique later. Cannon and gun barrels of cast-iron, bronze and brass were made using this method. It must be noted that casting technology was not fully developed and one of the major problems was that air bubbles trapped within the molten metal would leave hollows and cracks in the final product. To compensate for this, early gun barrels were made with very thick walls.

The next technique originated in Europe in the first half of the 14th century. In this method, the smith first makes a cylindrical pattern out of wood or clay, which is of the diameter and length desired. Next, the smith forges a number of iron bars of almost rectangular cross section and the same length as the pattern. These bars are fitted around the pattern as closely as possible, so there are no gaps between them. The pattern is then pushed out from the center, leaving the bars holding each other in place in the form of a tube. Then the smith forges a number of iron rings or hoops, whose diameter is slightly smaller than the diameter of the tube. The smith then heats these rings until they are white hot, which causes them to expand and they can be easily slipped around the iron bars. The rings cool and then contract, holding the metal bars in place.

For smaller pieces, instead of using bars of iron, the smith takes a flat rectangular iron sheet and bends it into a cylindrical tube and welds the long edge from end to end. Then he strengthens this tube by adding the metal hoops along the length of the tube, using the same technique explained above.

This technique of using straight staves and holding them in place with hoops was not a new idea. It was actually used centuries earlier for making wooden barrels to hold liquids. Hence, the name "barrel" also transferred to gun "barrels". This is true not only in the English language -- many other European languages also use the same words for "barrel" when referring to the wooden containers and the gun barrels.

So that's how the word "barrel" came to be used for guns as well.

Sunday, May 23, 2010

Rifling: Manufacturing: Electro Chemical Machining

In our last post, we talked about a modern manufacturing process called Electric Discharge Machining (EDM). In this post, we will talk about another process that also uses electricity, named Electro Chemical Machining (ECM).

This method is also a very precise method, but it is much more suitable for mass production. Like the EDM process, the ECM process can also be used on hard materials that cannot be machined by other more mechanical processes. Unlike the EDM process, no sparks are generated between the cathode and anode. The best way to understand ECM is to think of it as reverse-electroplating (i.e.) instead of adding material, we remove it.

Since 1993, Smith & Wesson has been using ECM to manufacture most of their revolver barrels. They use machines manufactured by Surftran to do their work. The barrels are hardened and annealed before the rifling process. The hardened barrels are then placed in the ECM machine and held stationary. The electrode is a plastic cylinder with metal strips circling around the exterior. The metal strips are a reverse image of the desired rifling and are inset into the plastic cylinder. This way, only the plastic part of the cylinder touches the barrel and not the metal strips. The electrode is placed inside the barrel and the whole is immersed into an electrolytic solution of sodium nitrate which is constantly circulating under pressure. The electrode is moved down the barrel and rotated at the desired rate of rifling twist. As current flows from the cathode (the electrode) to the anode (the barrel), the material is removed from the anode to duplicate the grooves in the shape of the electrode. Because the metal parts of the electrode never actually touch the barrel (only the plastic core does) and because the flowing electrolyte removes any material from the barrel before it has a chance to accumulate on the metal strips, the electrode usually lasts a very long time and needs no cleaning or maintenance. In fact, the electrode is replaced only when the plastic core which contacts the barrel to provide proper centering and spacing of the metal strips, wears out.

The advantages of this are that the process is extremely precise and can be used to machine hard materials like hard steel alloys, titanium alloys etc. Similar to the EDM process, it also produces no heat or stress on the barrel during the rifling process and also produces an excellent finish. Unlike the EDM process though, it is much faster to machine parts using this technique. A typical rifling job for a 357 magnum revolver barrel can be done in about one minute using this process, making it ideal for mass production. The tool can also be repeatedly used as there is very little tool wear.

The disadvantages are that these machines have high tooling cost and also use large amounts of electricity.

Rifling: Manufacturing: Electric Discharge Machining

In the last post, we studied a more modern method of adding rifling to a barrel, namely flow forming. Now we will study another modern rifling method called Electric Discharge Machining, otherwise commonly known as EDM. This method is also sometimes called spark machining, spark eroding or wire erosion.

The original EDM process was invented in 1943 by two Russian scientists, Dr. B.R. Lazarenko and Dr. N.I. Lazarenko. However, it was only in 1969 that the first commercial numerically controlled EDM machine was released by a Swiss company called Agie and the process was only fully refined in the 1980s.

The process consists of removing material from a workpiece by means of electrical sparks. The object to be machined (in this case, the rifle barrel) is connected to a source of electricity (the barrel is the "workpiece electrode") and another electrode called the "tool electrode" is placed close to the spot to be machined. Both electrodes are immersed in a dielectric liquid (typically, oil or de-ionized water). When the two electrodes are brought close to each other, the intensity of the electric field becomes higher than what the dielectric medium can contain and hence it breaks down, allowing current to flow between two electrodes. The generated sparks end up melting and vaporizing tiny pieces off both electrodes. Then the current is stopped, the old dielectric liquid is flushed out and replaced with new liquid, which also removes the metal debris out of the way. Now the tool electrode is positioned on the new area to be machined and the process is repeated. The entire process is automated and the tool electrode is moved by means of a computer numerically controlled (CNC) machine.

The advantages of using EDM for rifling are that it can be used on very hard materials that are otherwise difficult to machine by other methods. It is also very precise and there is no tool pressure so the rifling is not distorted. There is no heat or stress developed in the barrel as a side-effect of this process and no burring, so there is less post machining after the rifling is done. Most manufacturers use EDM machines not only for the rifling, but also to manufacture other parts of the rifle, such as the action and the receiver.

The disadvantages of this approach are that it is a pretty slow process and also consumes a lot of electricity. An EDM machine is also relatively expensive to purchase, hence this process is used by high-end custom manufacturers.

Friday, May 21, 2010

Rifling: Manufacturing: Flow Forming

In the last couple of posts, we studied two methods of adding rifling to barrels, namely Button Rifling and Hammer Forged Rifling. In this post, we will study a more modern method called Flow Forming.

Flow forming is a relatively new technique of metalworking that was invented in the 1950s in Sweden. It is a cold forming process that is used to make round seamless hollow components to precise dimensions. As it turns out, barrels are round (well, except for polygonal barrels, which can't be manufactured by this method naturally), seamless and hollow, so they fit the requirements to be manufactured by this process.

In flow forming, a hollow metal blank cylinder is slid over a mandrel made of some high tensile material. The metal blank is shorter and thicker than the final product. The outside of the mandrel has a negative image of the grooves, i.e. it has the raised groove reliefs carved on the outside.

In the above figure, the mandrel is the thin purple rod with the screw thread on the outside of it. Note that the mandrel is inserted into a metal cylinder blank that is initially much shorter and thicker than the final shape. Also note the three green rollers around the workpiece.

The mandrel and the metal blank workpiece are rotated and then the three green rollers apply pressure to the workpiece using hydraulics. The rollers are slowly moved outwards using CNC (Computer Numeric Control) to achieve precise dimensioning. Since the metal is compressed beyond its yield strength, it starts to flow and become longer and thinner. It also gets the impression of the rifling impressed onto its inside.
In the image above, you can see the green rollers being pressed onto the workpiece and you can see how the workpiece begins to elongate. The rollers deform the workpiece and lengthen it axially and also radially thin it simultaneously.

The rollers may be applied back and forth over multiple passes to elongate the metal workpiece to the desired length and diameter.
As you can see in the image above, the workpiece has reached its final dimension after a few passes. The workpiece can now be removed and further machined as needed. The following two pictures show the finished product:

As you can see from the above pictures, the finished product has a very high quality finish and a well defined set of rifling grooves. The original blank was 1/3rd the length of the finished piece and the whole process of elongating and rifling the metal blank to the piece shown above took only 30 seconds or so.

As you may suspect, even though the process is cold-forming, the application of pressure by the rollers tends to heat up the workpiece and hence a coolant fluid must be applied to the workpiece during the process.

The advantages of this process are many:
  1. The finish is excellent and dimensional accuracy is very high.
  2. The speed of manufacturing a raw metal blank to a final product is less than a minute.
  3. The hardness of the blank is increased by this process, similar to how Hammer Forging changes the hardness of the blank as well.
  4. Reduces the number of future operations to the barrel. For example, there is little to no grinding and polishing that needs to be done to the barrel, since the finish obtained by this process is so good.
  5. Since the rollers are applied to only a small section of the piece at a time, the pressure is very localized and there is sometimes a net savings of energy compared to other processes.
  6. Materials such as some grades of stainless steel, that are difficult to machine in some other techniques can be machined using this method.
The equipment tends to cost less than Hammer Forging equipment, but not as cheap as some of the other methods we've studied previously. The other issue to watch out for is that this method generates a lot of heat due to deformation and friction and hence, coolant fluid must be used to keep the workpiece cool during the process.

This technique is a modern way of manufacturing barrels and is gaining popularity in some sectors of the industry.

Thursday, May 20, 2010

Rifling: Manufacturing: Hammer Forged Rifling

In the last post, we studied the Button Rifling method. As was mentioned in that post, this is a method of manufacturing that is much more prevalent in the United States and was invented right in the middle of WW-II. Now we will study another method of rifling, that was also invented around WW-II, but by the other side. This method is called Hammer Forged Rifling and it was invented by the Germans in 1939. After the war, this method of rifling spread to all the neighboring European countries and this method is much more popular today in Europe than Button Rifling.

Right before WW-II, the Germans had invented a machine gun called the MG-42 (in 1939 actually, though it went into full production in 1942, hence the MG-42 name). This gun had an amazingly high rate of fire of over 1200 rounds per minute and it would need barrel replacements often, since the barrels would heat up so much. Hence, they needed a way to produce barrels at a much faster rate as well and the Hammer Forged Rifling method was invented. Interestingly, in the hammer forging method, the barrel is pounded into shape from the outside, whereas in button rifling, the barrel is shaped from the inside. The first hammer forging machine was invented in Erfurt, Germany in 1939 and these machines were shipped off to Austria ahead of the invading Russians at the end of WW-II, where American technicians got their first look at them.

The process starts by first starting with a barrel blank that is about 30% shorter than the final size and has a hole that's 20% larger than the desired size. Then a mandrel made of carbide is selected and inserted into the barrel. This mandrel has the negative image of the barrel (i.e.) it has the grooves carved in relief on the outside of the mandrel:

A mandrel for a .45 caliber rifled barrel. Note the rifling grooves in relief on the surface of the mandrel.

The mandrel is attached to a long thin steel rod and inserted into the barrel. The barrel is then surrounded by carbide hammers which pound it into shape from opposite sides, The barrel is then rotated a bit and then pounded again and the process is repeated until the desired shape is reached. The following two figures should give a very good idea of the process:

Isometric view of the process

Side view of the process

As you can see from the figures above, the barrel is hammered into shape a little bit at a time and the barrel is rotated and moved forward by the driver slowly. Since the mandrel and the hammers need to resist a lot of force, they are generally made of carbide. The hammers beat at around 1000 to 1500 times per minute and the whole process is very quick and takes only 3 or 4 minutes to make a barrel rifled.

As the barrel is being hammered, it reduces in diameter and increases in length. Hence the finished barrel ends up around 30% longer than the blank before the hammering process. The hammering process also leaves tiny dents on the outside (looks like snakeskin) which can be either left in place or cleaned up later, depending on the manufacturer.

As is the case with button rifling, this process induces a lot of stress of the barrel. Therefore the barrel needs to be stress-relieved right after the hammer forging process to prevent the barrel from splitting or deforming later. The process of stress relieving is the same as that done with button rifling (i.e.) heat the barrel to 525-575 degrees Celsius and then cool the barrel slowly.

The leading manufacturer of hammer forging machines in the world is GFM GmbH of Steyr, Austria. They not only manufacture hammer forging machines for rifling, but also for forging automobile parts in the European auto industry. Each machine costs over a million dollars and can spit out a new barrel in 3 minutes.

The advantages of this process are many: The process produces an exceptional finish on the end product. The accuracy of the bore and groove dimensions is fairly high and uniformity is maintained in the end product. The hammering process also work hardens the barrel. This process not only cuts the rifling in the barrel, but it can also take care of profiling the barrel and also shaping the chamber, all at the same time. This process is also used as the modern method to manufacture polygonal barrels. The speed of manufacture of barrels is very high, with a barrel produced every 3-4 minutes, which makes it very suitable for mass production

The main disadvantage of this process is the startup costs. There is a reason why this process is only used by large manufacturers, as each hammer forging machine can cost over a million dollars, which puts it out of reach of custom firearms makers, who are generally small shops. The second disadvantage is that this process puts stress on the barrel while manufacturing (even more stress than button rifling) which needs to be stress relieved carefully. Some materials (such as some stainless steel alloys) are harder to manufacture this way since they harden under hammering to such an extent that it becomes harder to further work them.

This process is generally wide spread in Europe because most European arms manufacturers are large companies and can therefore afford the high initial costs of purchasing a hammer forging machine. It also helps that the biggest manufacturer of hammer forging machines is in Austria, which is centrally located in Europe. SIG Sauer and Heckler & Koch from Germany, Sako from Finland, Glock and Steyr from Austria etc. all use hammer forged rifling. Prior to 1990, this method was not used by US manufacturers, but then Ruger got tired of buying barrels from subcontractors and decided to manufacture their own barrels (especially since there was a shortage of barrels then). Hence they purchased a few hammer forge machines from GFM of Austria and after their success, Remington purchased a few, along with some other manufacturers. This method is still considered a bit of a black art in the US, given that there are around 20-25 hammer forged rifling machines in the whole of the US. In fact, only eight people make all of Ruger's hammer-forged barrels.

Rifling: Manufacturing: Button Rifling

In our last post, we studied a method of rifling called Broach Rifling. Now we will study another method of rifling called Button Rifling. This is a method that was in development since the end of the 19th century, but wasn't really perfected until the 1940s or so by employees of Remington Corporation. It is the most common method of manufacturing rifled barrels in the United States today.

The button is actually a tool made of hardened steel or titanium carbide. It looks something like this:

The button is essentially a negative image of the barrel, i.e. the grooves to be cut are carved in relief on the button surface. There are two ways this button can be used: It can be pushed into the barrel or it can be pulled through.

The machinery to cut the rifling is very simple. The button tool is attached to a rifling head, which is in turn attached to a long rod of high tensile steel and the other end of the long rod is attached to a hydraulic ram. The rifling head is capable of rotating the button at a fixed rate, to produce rifling at a given twist rate as the button is being pulled or pushed through the barrel.

A Button Rifling Machine

In the above picture, you can see a barrel mounted in the middle of the machine. The right side of the barrel is blocked by a thick steel plate and on the left side, you can see the button about to be pulled into the barrel from left to right. On the right hand side, the pull-rod for the button is attached via a twisting spindle to a frame that has two hydraulic rams attached to it. Also on the right side is a rack and pinion gear system that drives the twisting spindle and twists the button as it is pulled through the barrel

The bore of the barrel is lubricated first, before the button is pulled through it. Each manufacturer has their own secret sauce as to what lubricant is used, and this is kept a closely guarded secret. As the hard button passes through the softer steel of the barrel, it engraves the rifling on to the inside of the barrel. The whole process takes only a minute or less. This is a cold-forming process, so there is no heat, but there is a lot of pressure involved (60,000 psi or 410,000 kPa is not uncommon), which is why this process requires a hydraulic ram. As you can imagine, this involves putting quite a bit of stress on the barrel, hence the barrel must be stress relieved after the grooving operation is completed.

The simplest types of rifling buttons merely cut the grooves into the barrel, and so these leave some burrs in the barrel after the grooves have been cut. Hence, the barrel needs to be finished separately if these cutters are used. Some other rifling buttons are combo buttons that have both a rifling button and a finishing button or sizing button following it. The rifling part cuts the grooves and the finishing part pushes the burrs back in and straightens out the edges caused by the rifling part of the button.

In the above image, we have two rifling buttons. The top one is a combo button which has a rifling button as well as a sizing button and the bottom one is a simple rifling button.

One school of thought believes that a pull button has a tendency to break off inside the barrel while it is being pulled through and hence, they use a push-button technique. In this case, the push rod is supported as it enters the barrel, to prevent buckling. The other school of thought believes that a pull button is best because it stays straight and true through the pulling operation, whereas with a push operation, the button has a tendency to yaw and cut a non-uniform thread. Needless to say, each school of thought has its own party of supporters.

The button must be made slightly larger than the desired bore and groove diameter of the barrel because steel is an elastic material and tends to spring back when it is pushed or bent. Hence if a groove of say 7.8 mm. diameter is sought, the grooves on the cutter would actually be slightly more than this (say 7.85 mm. groove diameter or so). Obviously, the properties of the steel used from batch to batch must be more or less uniform, otherwise the spring-back amount may be more or less and a new button will need to be made to compensate for this. Another interesting thing about this process is that it must be done when the barrel is in a blank state (i.e.) the barrel is an uniform cylinder. This is so that the forces resisting the button's movement are uniform throughout the barrel. Otherwise the rifling will be uneven inside the barrel.

As mentioned earlier, after the grooves have been cut, the barrel must be stress relieved because the process puts a lot of stress on the steel. If this is not done, the barrel may split or deform later when the weapon is fired. Another reason to do this is because after the grooves are cut, if the barrel is further machined on one side (e.g. the muzzle end is slightly thinned and it is machined to fit the chamber), the barrel will deform on that side due to the stresses from the button rifling operation, if the stress isn't relieved first.

Stress relieving is done by heating the barrel in a furnace to about 525-550 degrees celsius and then slowly cooling it. The bore may slightly snap back during the stress-relieving process, which is why the button is slightly larger than the desired size. Even though the barrels have been stress-relieved, it is not possible to do it completely without compromising the hardness of the barrel's steel. Hence, when the barrel is machined again, the bore may open up slightly on that side, even if the machining is done on the outside of a barrel. Barrel makers usually use a process called lapping (which we will discuss later when discussing the art of barrel making) to restore the barrel back to a uniform bore.

There are a lot of variables that affect the bore and groove dimensions with button rifling: the particular batch of steel involved, its hardness, size of the bore, size of the button, how fast the button is pulled through, how much the bore closes after stress relieving and how much it opens after final machining etc. This sounds like a lot of things can go wrong, but a skilled and experienced barrel maker knows how to balance all the factors out to make the correct dimensions. Still, at the end of the day, the barrel maker only knows what the final exact dimensions of the bore will be, after the barrel is fully machined. This is why some barrel manufacturers offer different grades of button-rifled barrels.

Button rifling is very common among most barrel makers in the United States. In fact, it is the most used method today, because of the small amount of time needed to make the rifling, and the fact that if the barrel-maker is experienced, the dimensions of the final product can be very accurate as well. The tooling and machinery used to produce button rifling is also very cheap. This is why this method is used a lot in mass production.

On the other hand, the disadvantages are that the material of the barrel needs to be very homogeneous and uniform hardness throughout the bar, otherwise the cut will be uneven in the softer parts of the barrel vs. the harder parts. The exact twist of the rifling is also somewhat unpredictable. If the button should slip inside the barrel, you may get a barrel with (say) 1 turn in 205 mm. when the desired rifling is 1 turn in 200 mm. Another problem is that the groove may not be centered (i.e.) one side might have deeper grooves than the other side due to non-uniform hardness of the barrel.

Compared to the other two methods we studied earlier, this method offers more disparate results. This is why manufacturers offer different grades of button rifled barrels. The ultra premium quality button-rifled barrels cost a lot more than the standard and sporting quality button-rifled barrels.

Wednesday, May 19, 2010

Rifling: Manufacturing: Broach Rifling

In our previous post, we studied a manufacturing technique called Cut Rifling. As we noted in the previous post, Cut Rifling produces very accurate results, but it is very slow to manufacture as the rifling is cut little by little. Now we will study a more modern technique called Broach Rifling.

A broach is merely a long tool with multiple cutting elements on it. A broaching tool may be pulled or pushed along a workpiece. It must be noted that each cutting element on the broach tool is slightly larger than the previous one.

The image above shows a typical rotary pull-type broach tool. The tool is placed inside an item to be cut and then rotated and pulled through the item. The tool has a set of roughing teeth that make the initial rough cuts, followed by a set of semi-finishing teeth that make finer cuts, followed by a set of very fine finishing teeth that leave a very good finish. The diameter of the rear pilot section of the broaching tool is a little less than the diameter of the finishing teeth. Making a good broaching tool is key to this technique and the broach tool is unique in that rough-cuts, semi-finishing and final-finishing is all done with the same tool.

The process of broaching was invented in the 1850s or so, but it was only applied to making rifling after WW-I ended in 1918. Advances in broaching machines have made this method very competitive with other machining processes.

Now we will study a specific broach tool used to manufacture rifling. This tool is a hardened steel rod with multiple cutting disks spaced evenly on the rod. Each cutting disk has multiple cutting elements, as can be seen in the two pictures below:

This type of tool is called a gang-broach because it has multiple cutting rings. Each ring is of a slightly larger diameter than the previous one and has multiple cutting edges. The last ring is of the desired groove diameter of the barrel. The rod is twisted at an uniform rate as it is being pulled through the barrel. This provides the spiral rifling grooves. Since each ring has multiple cutting edges, all the grooves may be cut at the same time and with a single pass of the broaching tool.

Broach cutting works similar to the Cut Rifling process we studied in the previous post, except that there is no need to make multiple passes to get the desired groove depth. The entire job can be done in a single pass and provide quite accurate results. Quite a few barrel manufacturers use broach cutting to rifle their barrels.

The disadvantage of broach cutting is that it requires some skilled workers to make the broach tools. Also, a broach tool is always designed for a very specific job and cannot be reused to make a different barrel, say one with more grooves or a different diameter etc. For each job that has a different diameter or number of grooves, a separate broach tool must be manufactured first.

Rifling: Manufacturing: Cut Rifling

In all our previous posts, we've studied the history of rifling in guns. Now we will study some of the manufacturing techniques of rifling. The first method we will study is called Cut Rifling, or more accurately, Single Point Cut Rifling.

This is one of the oldest methods of rifling and dates back to the time that rifling was first invented in Nuremberg, Germany in 1520. Cut Rifling consists of removing steel from the inside of a barrel using a cutting tool with a hard point. The cutting tool used is called a "hook cutter".

Image of a hook cutter

The cutting tool is attached to a cutter box, which contains a mechanism to raise the cutter. The cutter box is cylindrical in shape and is made smaller than the bore of the barrel, so that it may be inserted into the barrel as well. The other end of the cutter box is attached to a hollow steel tube through which coolant fluid (usually oil) is pumped to keep the cutting head cool. As the cutter box is pulled through the barrel, it is also rotated about its axis at a set rate to give the rifling a spiral shaped groove.

The hook cutter mounted into a cutter box. The hook cutter is in the middle of the cutter box and
the sharp edge is exposed via the groove in the cutter box. The hook cutter sits on a wedge and when the
thin screw at the end of the cutter box is turned, it pushes the wedge forward under the cutter, thereby
raising the cutter outwards to increase the depth of the cut.

Each time the cutter box is pulled through the length of the barrel, it only removes a very small amount of metal (typically about 0.002 - 0.005 mm or so). Say we wish to cut 3 rifling grooves in the barrel. The cutter is initially positioned so the cutting point is in the far end of barrel and pulled through the barrel and rotated simultaneously to make the first groove. Then the cutter is again pushed back into the barrel and at its starting point, it is rotated 120 degrees (by using an indexing gear to control what angle the cutting starts at) before starting again. Then after the second cut is made, the cutter is again pushed back in, but it is first rotated 240 degrees before starting the cutting operation again. After the 3 initial grooves are made, the cutter is moved back in, but now the cutting depth is increased by 0.002 mm, so when it cuts the first groove again, it will now cut deeper. Again the three grooves are cut, then the cutting depth is increased again by 0.002 mm and then the three grooves are cut and so on, until the desired groove diameter is reached. As you can see, this operation is very tedious and can take a few hours to finish.

Most of the machines that are commonly used to manufacture this type of rifling date from about the late 19th or early 20th century and quite a few of them are still found in perfect working condition! A majority of the machines used in North America are made by a company that is now very well known for aircraft engines: Pratt & Whitney. It must be noted that even though Pratt & Whitney is now a major aero-engine manufacturer, the founders, Francis Pratt and Amos Whitney, originally met when they were both working in a revolver factory owned by Samuel Colt! The following picture is a P&W Sine bar cutter made in 1895!

It is called a sine-bar cutter because a sine-bar is used to control the uniform rotation of the cutter box as it is being pulled through the barrel. These machines have a pulley-and-belt driven mechanism and are made for the smaller workshops of the era. It only weighs about one ton or so and can be mounted to a wooden floor. Thousands of these were made to meet the demand for barrels in World War I and after the war, many of them were sold off for cheap in the second-hand market and ended up in small custom barrelmakers' shops where they are still used to this day.

At the end of the 1930's, Pratt and Whitney introduced a heavier hydraulic driven "B-series" of rifling machines. This was basically two machines mounted side by side on a single bed and weighed about 3 tons. Since this was a much heavier machine, it was only suitable for larger factories with concrete floors. These were much more rigid than the sine-bar cutters and more powerful as well. Only a few thousand of these were made and after WW-II, many of these were scrapped since they couldn't be sold as easily as the earlier model. Due to these factors, "B-series" machines are actually rarer these days than the sine-bar cutter machines.

A Pratt & Whitney "B-Series" Hydraulic Rifling Cutter Machine. Note that this machine actually has two sets
of machinery which are mounted parallel to each other. This allows the operator to cut two barrels at the same time.

During WW-II, many better methods of making rifling were invented and hence, the B-Series machines were the pinnacle of Cut Rifling technology. However, there are still some custom barrel makers that use these machines to this present day. These makers are usually specialist makers who only make one or two barrels to very exacting specifications.

Cut Rifling is slow in nature and is labor intensive. The price of a rifling cutter machine is not very expensive and small shops can afford to buy them. The issue though is that they haven't been manufactured in many years, so finding one for sale is a bit of a problem, as is finding spare parts. Some small shops get around this problem by making their own homemade machines, usually by converting a lathe. Rifling cutter machines also need a high level of technical skill to maintain the tooling (i.e. build new hook cutters, replace cutter boxes etc.). On the other hand, making the tooling is relatively cheap and the tooling is reusable. For instance, the same hook cutter and cutter box can be used to make different barrels all of which have a different number of rifling grooves of differing depths and twists. This makes it very suitable for custom barrel-makers who make only a couple of barrels at a time, to very specific customer requirements. Since custom barrel-makers make their products to very precise specifications, they also charge higher prices for their barrels.

The results obtained from Cut Rifling are very accurate: the groove diameter and twist are very uniform and it also doesn't put any stress on the steel. Some other methods of making rifling need the steel to be of a certain quality to work, but Cut Rifling can be used on a wide variety of steel grades with no problems whatsoever.

The main disadvantages with this method are that it is not really suitable for mass-production compared to some of the other newer methods of rifling, and that it requires highly-skilled people to operate the machines and make the tooling.

We will study more methods of manufacturing rifling in the next few posts.

Tuesday, May 18, 2010

Rifling: Terminology

In the previous post, we've studied the rifling for breechloaders. We also studied polygonal rifling earlier. We will now study some basic terms before we go on to some manufacturing technologies for rifling.

As you can imagine, due to the rifling of the barrel, the cross section of a barrel is never truly circular. It may be circular with grooves cut into its edges, or it may be polygonal (usually hexagon or octagon) with rounded edges. Hence the diameter of the barrel is commonly expressed in two ways:
  1. Bore diameter - This is the diameter of the barrel without considering the depth of the grooves
  2. Groove diameter - This is the diameter of the barrel also considering the depth of the groove.
The picture below should make things clearer as to which is which:

The above picture assumes a traditionally rifled barrel (i.e.) a barrel which is circular in cross-section with grooves cut into it. In the case of a polygonal barrel (let's say it is a hexagon in cross section), there will also be a minimum width and maximum width. The minimum width is the distance between two opposite sides and the maximum width is the width between opposite edges.

Because there are two different diameters in a rifled barrel, this sometimes complicates matters when expressing a rifle bore. The hugely successful Enfield .303 rifle that is still popular with railway security men in India is named that way because has a .303 inch bore diameter (i.e. 7.7 mm). However, if the groove diameter of this rifle is measured, it is actually 0.311 inches (i.e. 7.9 mm) in diameter because the groove has a depth of 0.004 inches on each side. Since it is a breechloading rifle, it fires a bullet that is slightly larger than the groove diameter of the barrel. The diameter of the bullets used by this rifle are usually 0.312 inches and when the gun is fired, the bullet gets slightly deformed as it enters the barrel and engages the grooves.

Another term which we will encounter frequently is the twist of a barrel. This is basically a measure of how quickly the groove makes a complete turn in the barrel. For instance, in an AK-47, each groove makes one complete turn for every 237 mm. of barrel length, in a M16-A2 assault rifle, it is one complete turn every 178 mm. and so on. From this, we can say that a M16-A2 barrel has a tighter twist than an AK-47.

Rifling: Breechloaders

In all the previous posts in this section, all the weapons have been muzzleloaders. We will now move on to discussing breechloading rifles. All muzzleloading weapons, by their very nature, require a user to stand up to load the weapon; and also require more steps to complete the loading procedure. This is where breechloaders have an advantage because the user can shoot much faster with a breechloader weapon.

The main problem with adopting breechloading technology was how to make the breech gas tight, so that it would not leak gases out of the sealed end of the barrel. One of the early breechloading weapons was actually a rifle -- The Ferguson Rifle, which we already discussed in the breechloading article linked above. However, it was not until the mid 19th century, when manufacturing technologies were sufficiently advanced, that the breechloader was considered again. We've also seen some of the 19th century breechloaders while discussing the needle gun cartridge. Both the Dreyse Needle Gun and the Chassepot were also rifled arms.

In breechloading weapons (including modern pistols, rifles etc.), the cartridge is loaded into a chamber at the breech (i.e.) close to the end of the barrel near the trigger. The chamber is somewhat larger than the bullet, so that the cartridge can be easily loaded into it. The chamber then narrows down to a throat which connects to the barrel. Normally, the diameter of the throat is roughly around the same diameter of the barrel with the grooves. The bullet is usually slightly larger in diameter than the diameter of the barrel with the grooves. When the weapon is fired, the bullet gets slightly deformed as it moves through the throat and enters the barrel. The deformation makes the bullet engage the grooves of the barrel and it starts to spin as it makes its way out of the barrel.

As black powder gave way to more powerful explosives such as cordite, the velocity of bullets increased and so it was no longer possible to manufacture them out of soft lead. Hence, we now have harder lead and copper jacketed bullets. These are made to prevent the bullet from being stripped by the grooves as it makes its way through the barrel.

As the bullets make their way out of the barrel, they tend to wear out the rifling. The maximum wear is usually around the chamber throat area where the chamber transits into the rifled barrel, as the pressure is highest in this area. Because heat can also wear out and deform the rifling, many light, medium and heavy machine guns have quick-change barrels, so that one barrel can be given sufficient time to cool down while another is used to fire the bullets.

Monday, May 17, 2010

Rifling: Polygonal Bore and the Whitworth Rifle

In our last post, we studied the principle of rifling with the expanding bullet. Now, we will study another system of rifling that has also stayed with us to this present day. We start our history with the Enfield 1853 rifle, which worked with an expanding bullet (the Minie ball concept) that we talked about in the previous post. While the original design was done by Enfield, the actual manufacturing of weapons wasn't always done by them, since they didn't have the facilities to produce the weapons at the rate that the British military needed them. Hence, several London based firearm manufacturers were subcontracted to manufacture the weapon to the Enfield design's specifications and the job of checking that each contractor manufactured to the same specifications fell to the Royal Armory at the Tower of London.

In 1852, the official Master of Ordinance and Arms, whose job it was to ensure that standards of weapons manufacture were met, was Sir Henry Hardinge, First Viscount Hardinge of Lahore. Indian readers of this blog might recognized the name because this is the same Hardinge who had earlier served a term as Governor General of the East India company, founded Roorkee University (now IIT Roorkee) and under whose term of rule, the first Anglo-Sikh war had taken place. He had returned to England after his term in India and had been promoted to the post of Master of Ordinance and Arms. Viscount Hardinge was engaged in the study of the comparative merits of the rifles manufactured by Government owned factories (such as Enfield Rifle works) and those made by private manufacturers. He was most distressed to find that there were such wide standards used in manufacturing that no two rifles shot alike and many parts could not be interchanged, even if the rifles were the same model.

In order to solve the problem, he recruited Mr. Joseph Whitworth (later Sir Whitworth), to improve the precision of manufacturing tools. Mr. Whitworth was arguably the foremost mechanical engineer of that time period and was already famous for manufacturing precision tools (such as the Whitworth lathe, shapers, planers, drills etc.) and establishing a standard for screw threads (The British Standard Whitworth (BSW) thread standard, which is still used to this day. E.g. look at the base of any camera where you attach a stand and that's a whitworth screw thread). After some research, Mr. Whitworth came to the conclusion that the fault was really in the design of the rifle rather than the production techniques, and was commissioned by the British Government to improve the rifle design altogether. He was built an extensive test facility for this purpose, which became operational in 1855. Meanwhile, Mr. Whitworth had already built a cannon using a polygonal bore in 1854 and decided to adopt this same principle to firearms. He also worked to determine the best form of bullet.

At that point, the bullet that was being used by the Enfield Model 1853 rifle was cylindro-conoidal and on attempting to elongate the bullet, he discovered that it had a tendency to tumble over in flight. He determined that this was due to the slow spiral of the rifling (the Enfield had a 39 inch barrel with three threads, each making one turn in 78 inches) and tried out every combination from 1 turn in 5 inches to 1 turn in 78 inches to find the best possible combination. He eventually settled on a rifling of 1 turn in 20 inches, which he determined gave the bullet enough stability in flight to not tumble over, but also kept the forward velocity of the bullet high enough.

Whitworth was already known for inventing Engineer's Blue, which was useful for machining perfectly plane surfaces (i.e. flat surfaces), and several precision planers. Hence, he determined that instead of a round rifled barrel, he was going to manufacture one using several plane surfaces (i.e. flat surfaces). The reason for this was because he could machine flat surfaces much more precisely and thereby increase the tightness of fit of the bullet. The bullet could be made of harder lead material as it did not need to deform to engage the rifling. In the process, Whitworth elongated the bullet (at that point, Minie balls were still more round than tapered) and made it more like the modern bullets of today. He also discovered that with each variation of bore, the charge and size of the bullet had to be modified as well and determined that 0.450 inch diameter was the optimum for the amount of gunpowder and weight of lead that he was restricted to (the restriction was that the weight of gunpowder could not exceed 70 grains and the weight of the bullet could not exceed 530 grains)

First, we will look at the competing Enfield bullet and rifle bore:

The Enfield 1853 rifle is a muzzle-loading weapon that has a barrel that is circular in cross-section and is 39 inches long with 3 grooves in it. Each groove is cut so that it would make a complete turn every 78 inches, which means that each groove makes one 1/2 turn through the length of the 39-inch barrel. The bore of the Enfield is 0.577 inches and the length of the bullet is 1.8 times its diameter. The bullet is cylindro-conoidal in shape and has a wooden plug at the base. The bullet is wrapped in greased paper before being inserted into the barrel. Firing mechanism is a percussion lock and upon firing the weapon, the bullet expands and engages the three grooves of the rifling. In order for the expansion to happen properly, the lead is required to be very pure and soft.

Now we look at the new Whitworth design:

The Whitworth rifle is also a muzzle-loader with a percussion lock firing mechanism. Like the Enfield 1839, this model also has a 39 inch long barrel, but it has a smaller bore of 0.451 inches, since Whitworth's experiments proved that this was the best caliber for the gunpowder amount and bullet weight requirements. The rifling is much tighter, being one turn in 20 inches, which means the bullet makes almost 2 turns by the time it comes out of the barrel. The cross section of the barrel is hexagonal instead of circular and the smallest diameter is 0.451 inches and the widest diameter is 0.490 inches. The bullet itself is smaller in width and is also hexagonal shaped in cross-section. However, it is much longer than the Enfield bullet, with the length to width ratio being 3 : 1. The bullet can be dropped into the barrel much easier than the Enfield bullet. Since expansion of the bullet is not critical to engage the rifling, it is possible to use bullets made of harder materials, e.g. tin-lead alloys or hardened steel! An expanding bullet made of soft pure lead may also be used as well.

It was argued that the tighter rifling of the Whitworth rifle (1 in 20 inches) reduced the velocity of the bullet, but a test conducted in 1857 at Government School of Musketry at Hythe showed that the Whitworth rifle penetrated fifteen elm planks to the Enfield's six, with the same amount of gunpowder in each. A hardened bullet on the Whitworth penetrated 35 planks, whereas the Enfield, which could only shoot soft lead bullets, only managed to penetrate 12 planks. The rifles were also put to accuracy tests by taking ten shots each at targets at various ranges. The following table and diagram illustrate clearly the results that were obtained during the trial (figures reproduced from W.W. Greener's The Gun and its Development, Second Edition):

6.20 to 7.0
No hits
Shooting was so wild that Enfield was not even considered.

As you can see from the figures and the bullet holes above, the Whitworth rifle clearly outperformed the Enfield 1853, which was the premier rifle of its day. Until the Whitworth came out, the best recorded deviation from the center for any rifle at 500 yard ranges was 27 inches (2.24 feet). The Whitworth rifle at 500 yards had a mean deviation of only 4.5 inches! At 1400 yards range, the Enfield could not even hit the target square which was 14x14 feet in size.
The committee, while acknowledging the Whitworth's superiority in accuracy, also noted that the barrel tended to foul up easily due to the quality of black powder used in those days. They also expressed doubts about how the hexagonal bullets could be manufactured in bulk and had concerns about whether these rifles could be manufactured with equal mechanical perfection. At that time, the British tactics relied on mass-rapid fire and less on accuracy and the Whitworth rifle also cost 4 times more than the Enfield to manufacture. As a result, the Whitworth rifle was rejected by the British Government as non-suitable for their order of battle. There was a trial held in England in 1860 at the National Rifle Association, but since Whitworth was not a gunmaker of experience, his rifle had ignition problems due to faulty percussion-lock design. On the other hand, rival gunmakers learned from his methods and produced weapons of similar caliber, using the same rifling, but better firing mechanisms. The Henry rifle which won the competition had many of the features pioneered by the Whitworth rifle. One more problem with the Whitworth was that it was a muzzle-loader and hence shared the defects of all muzzle-loaders (i.e.) took longer to load than a breech-loader and the user had to stand up to reload the weapon.

However, this did not spell the doom of the Whitworth rifle. There were other people who were interested in accurate weapons -- quite a few Whitworth rifles were purchased by the Confederate Army of America and were used in the US Civil War with deadly accuracy. The Confederates were fighting against an Union force that was vastly larger and had better artillery. To counter the artillery of the Union army, the Confederates created a unit called the Whitworth Sharpshooters which was designed to harass the artillery batteries. In fact, this unit was the birth of the first dedicated modern sniper unit and the Whitworth is one of the original sniper rifles!

Traditional rifling is still used for many sharpshooter weapons, but polygonal rifling has the advantage of higher velocities, less wear and tear than conventional rifling and less lead and copper buildup in the barrel, which makes cleaning easier.

Due to the success of the polygonal bore, many manufacturers still manufacture weapons using this technique. While the polygonal bore idea fell out of fashion between 1890 and 1940, the Germans brought the concept back with the MG-42 machine gun in WW-II. The well-known Heckler and Koch G3 assault rifle also has polygonal rifling (from the G3A3 model upwards), as does their PSG-1/MSG90 sniper rifle. Polygonal rifling is also used on some handguns made by Glock, Heckler & Koch, Ceska Zbrojovka (CZ), Kahr arms, Desert Eagle and others.