Headspacing - II

In our last post, we looked into the subject of "headspacing", we will continue to study our subject in this post. As was suggested in our last post, headspace is the distance between the bolt face and a point in the chamber that prevents further forward movement of a cartridge. Depending on the firearm and the shape of the cartridge, this distance can be different for different types, as the figure below shows:

Click on image to enlarge.

Image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported by user Amendola90 at wikipedia.org

Now we will look into what happens if the headspacing is not correct. There are two scenarios we must consider here. The first is excessive headspacing, where there is extra space between the bolt face and the cartridge in the chamber. The second is insufficient headspacing, where there is too little space between the bolt and the cartridge.

What if there is excessive headspacing? In this scenario, when the firing pin hits the cartridge, it will move forward into the chamber before detonating. When the propellant explodes, the walls of the cartridge will expand due to heat and firmly stick to the walls of the chamber, preventing rearward motion of the cartridge. However, the thicker base of the cartridge will move backwards, because there is now a gap between it and the bolt face and this will cause the walls of the cartridge to stretch. If the stretching is too much, the walls of the cartridge could rupture and release hot gases into the action and also potentially spray case fragments out from the action of the firearm, which could be hazardous to the shooter or people standing next to him.

Well, what if there is insufficient headspacing in the chamber? In this scenario, the back of the cartridge will stick out of it and the user will not be able to close the bolt fully on the loaded cartridge. The user will not be able to properly operate the firearm if this happens. If the user were to force the bolt to close on the cartridge, this pushes the bullet tightly into the case neck and if the firearm is fired, this will cause excessive pressure to build up inside it, leading to hot gases coming out of the cartridge's primer pocket with similar results as excessive spacing. In worst case scenarios, the excess pressure could cause the action to rupture and cause damage to the gun and its user.

So, how do we determine that a firearm has proper headspacing? We can do that by using a set of gauges, called headspacing gauges. These are measuring instruments that are precisely machined to the SAAMI or CIP or military standards of a cartridge caliber. Typically, these are made of heat-treated steel and are machined to tolerances below 0.001 inches or so. They are made for various calibers by several reputable commercial companies at fairly reasonable prices. Typically, there is a "Go" gauge, a "No-go" gauge and (for military specification rifles), a "Field" gauge.

Headspacing gauges. Top is a "No-go" gauge and bottom is a "Go" gauge

The "Go" Gauge: A firearm must be able to close the bolt with no resistance at all, when a "Go" gauge is inserted into the chamber. This signifies that the firearm is able to meet the minimum length specification for the cartridge. If the bolt does not close on the "Go" gauge, this means that the firearm has insufficient headspacing and may be dangerous to fire, even if the user manages to force standard cartridges to fit in the chamber. Sometimes, the cause of this may be a dirty chamber or bolt face, because the accumulated dirt may be thick enough to prevent the bolt from closing on the gauge. However, if the firearm is clean and the bolt still does not close on the "Go" gauge, it must be taken to a competent gunsmith for adjustments.

If a firearm successfully closes on a "Go" gauge, it means that the firearm at least has sufficient headspace, but it may still have excessive headspace. That can be determined with our next gauge.

The "No-go" Gauge: A new (or overhauled) firearm must not be able to close on a "No-go" gauge. If the bolt closes successfully on a "No-go" gauge, this means the firearm has excessive headspace and there is a risk of cartridge cases rupturing inside the chamber. If the firearm is new or recently repaired, that means it should be returned to the manufacturer immediately.

A used firearm may be able to close on a "No-go" gauge, due to wear of the bolt and chamber surfaces, but this means that it should probably go to a gunsmith for repair soon and it may be possible to fire new factory ammunition in it until then, but reloaded ammunition is probably a bad idea and the firearm may malfunction on slightly out-of-spec cartridges. Here's where the test with the third gauge comes in (especially for firearms built to military specifications).

The "Field" Gauge: The bolt of any firearm, whether old or new, should not be able to close on a "Field" gauge. A bolt that closes on "no-go", but not on a "field" gauge, may be considered close to being unsafe, but may work on new cartridges (and should be sent to a gunsmith soon for possible re-headspacing). However, if the bolt closes on a "field" gauge as well, then it is definitely not safe to fire and should be sent for repairs immediately (or mounted on a wall as a decorative piece!).

Some calibers have a fourth gauge called a "Field II" gauge, for which the bolt should never lock on. This type of gauge is only used by some rifles (for example, Colt uses it to reject M-16 rifles).

It must be noted that gauges are usually manufactured to either SAAMI, CIP or military standards and therefore may have different dimensions, even for the same caliber cartridge. Therefore, it may be possible that a rifle manufactured to NATO specifications, may lock on a "No-go" gauge built to SAAMI specifications, but correctly not lock on a "No-go" gauge built to NATO specifications. This is because military weapons are generally designed to operate with wider tolerances and military ammunition cases are generally thicker than commercial ammunition and can tolerate more stretching without rupturing. Therefore, a military firearm may fail the test using SAAMI gauges, but still be deemed somewhat safe to fire per the military specification gauges. However, if it passes even with using SAAMI gauges, that means it is very likely to work correctly.

Go, No-go and Field gauges are available from many firearm supply stores and cost about $25 to $50 per gauge or about $70 to $140 per set.

Headspacing - I

It has been a while since the last article. Today, we will study an important concept called "Headspacing". This will be a two-part post.

The term "headspacing" is defined as the distance between the face of the bolt and a point in the chamber that prevents further forward movement of a cartridge.

Click on image to enlarge. Public domain image.

In the above pictures, we have a .45 ACP cartridge loaded into a chamber. The cartridge case is a bit larger than the bullet and the chamber is shaped so that it fits the case correctly. Of course, depending on the shape of the cartridge, the headspacing could be different for different firearms.

For instance, some of the earliest metallic cartridges were rimmed. This means, they have a rim at the base of the cartridge that is larger than the diameter of the cartridge case. When this type of cartridge is pushed into a firearm's chamber, the rim positions the cartridge correctly and prevents it from slipping too far into the chamber. Therefore, for a rimmed cartridge, the proper headspacing should be the width of the rim.

Headspacing of a rimmed cartridge. Click on image to enlarge.

The most popular cartridge today, the .22 Long Rifle rimfire cartridge is an example of a rimmed cartridge.

However, it must be noted that most modern cartridges are rimless (i.e.) the diameter of the rim is the same or smaller than the diameter of the case body. Modern cartridges have other ways to ensure that the bullet seats in the chamber correctly. For example, for most pistol cartridges, the case diameter is slightly larger than the bullet and the chamber has a shoulder that the case mouth rests against.

Headspacing on a .45 ACP rimless cartridge. Click on image to enlarge.

In the image above, we see a .45 ACP rimless cartridge seated in a chamber. The case mouth is larger than the bullet diameter and rests against the front of the chamber, thereby correctly seating the cartridge.

Other rimless cartridges, such as bottle-necked cartridges (which are mostly used by rifles), the shape of the firearm's chamber is correspondingly tapered to achieve proper headspacing.

Headspacing on a bottlenecked rimless cartridge. Click on image to enlarge.

In the above image, we see that the chamber is tapered to accommodate the cartridge and the headspacing is the distance from the bolt face to the tapered section.

In the case of belted cartridges (such as the hunting cartridge, Holland & Holland .300 Magnum), the chamber is shaped to seat the forward face of the belt.

Headspacing on a belted cartridge. Click on image to enlarge.

When the H&H .300 Magnum was first being developed, it was difficult to provide proper headspacing for a cartridge with a shallow shoulder design. The solution was to add a belt around the cartridge body. This is similar in function to the rim of a rimmed cartridge, but gave a long enough surface for the cartridges to fit side-by-side in a magazine, without risk of interference during the feed stroke. This idea was later copied by other magnum rifle cartridges as well.

We will study the importance of proper headspacing in the next post.

Unusual Firearms: Key Guns

In our last article about unusual firearms, we looked at the Greener police shotgun. In today's post, we will look at a very unusual class of firearms, a combination of a key and a gun, the so-called Key Gun.

Key guns were apparently used in the 17th and 18th century, but they are pretty rare. They were apparently intended to be used by jailers for self-defense purposes, when they were making their daily inspections of the prison cells. The idea being that the jailer could use the same object to open the cell door and also defend himself, in case the prisoner inside tried to escape.

Key guns were initially incredibly primitive devices. The example above is merely a tube sealed on one end, with a touch-hole on top. Note the lack of any firing mechanism. The jailer would have carried a lit cigarette or cigar in his mouth. To discharge this weapon, the jailer would touch his cigar on the touch-hole and light the powder within the key.

Some other examples had actual firing mechanisms. For instance, we have a model that uses a matchlock firing mechanism, one using a flintlock and another one using percussion cap technology.

Click on images to enlarge.

Whether these weapons were actually practical or not is a completely different matter. These are all small-caliber and short range firearms with low stopping-power and it isn't quite clear how usable one of these is once the jailer has started turning the key, because the gun can no longer point at the prisoner and the jailer can't put his finger on the trigger while turning the key. No wonder then that these weren't very popular.

Corrosive Ammunition

Many months ago, we talked about the development of the percussion lock. This was where the idea of striking a shock-sensitive substance with a hammer to ignite the main charge of propellant was originally developed. In that article, we had mentioned that mercury fulminate was originally used as the primer and was later replaced by potassium chlorate. This idea of using a shock sensitive priming material is still used in today's centerfire cartridges. However, there were some issues with using such primer materials and we'll study about them in this post.

In our discussion about the percussion lock previously, we'd mentioned that the inventor (the Rev. Alexander Forsyth) had used mercury fulminates to set off the main charge. Mercury fulminates continued to be used in priming caps for early centerfire cartridges as well, into the end of the 19th century. However, when people started to switch to using smokeless powders, they began to discover the downsides of mercury fulminate. One of the issues was that mercury fulminate tended to degrade when kept in storage. This was not really an issue when using black powder cartridges, because black powder ignites a lot easier than smokeless powders. However, once people started to switch to smokeless powders for extra power, they found that keeping the cartridges in storage would cause the mercury fulminate primers to degrade so much that they could not reliably ignite the smokeless powder, causing misfires and hang fires. One more problem with mercury fulminates was that in conjunction with smokeless powders, it tended to form copper and zinc amalgams in the brass cases of cartridges, thereby making them unsuitable for reloading.

Due to this, the US Army switched to using potassium chlorate primers in 1898. Some other manufacturers used sodium chlorate instead. While these primers did not degrade as much as mercury fulminate, there were some other problems that came with them. When fired, these primers would decompose and leave a residue of potassium chloride (or sodium chloride) behind in the barrel. Those of you who remember your chemistry lessons in school might remember that sodium chloride is the scientific name for common salt. Potassium chloride is also another corrosive salt. These salts are highly hygroscopic (i.e.) they tend to attract water, especially when in humid conditions. Guess what happens when you have salt and water applied to an iron or steel surface -- that's right, it rusts. Therefore, if the barrel and action are not cleaned after firing such cartridges, there's a good chance that they could rust soon after. Swabbing the surfaces with oil will not prevent these salts from attracting water and rusting the metal.

As a result of this, primers using non-corrosive chemicals were developed in the 1920s, but these were generally used in civilian ammunition only, as the early non-corrosive primers did not last as well in storage as corrosive primers. Due to this, military ammunition tended to use corrosive primers and this was indeed the case for US military ammunition until the 1950s or so. Some other countries (e.g. former Soviet Union, China, Yugoslavia, Bulgaria etc.) continued to use corrosive primers in their cartridges for much longer than this, well into the 1970s and 1980s. Therefore, depending on the source and the age of the ammunition, the user must be wary lest the ammunition is corrosive.

So how does a user ensure that his firearm doesn't rust after using corrosive ammunition. The good news is that this is fairly easy to handle. It turns out that these corrosive salts dissolve in water. Therefore, cleaning the firearm thoroughly using water or a water based lubricant should do the trick. Some people use hot soapy water, others use plain water, still others swear by windex glass cleaner (which is largely water based). After washing off the residue, the firearm should then be dried, then cleaned with normal bore cleaning solution and oiled, as per the normal cleaning procedures.

The firearm should be thoroughly cleaned as soon as possible after firing the corrosive ammunition, to ensure that the corrosive chemicals are removed before they can damage the firearm. While this may seem like a bit of extra work, it is well worth it because there is no way to restore a barrel or chamber back to perfect condition, once it has started to rust. Therefore, the user should go through the extra effort if the ammunition is suspected to use corrosive primers.

As it turns out, the price of surplus ammunition using corrosive primers is often much lower than other types and it is also widely available in the market. So how does a user identify if the ammunition is corrosive or not? One way is by looking at the markings on the cartridges and the manufacturer of the ammunition. For instance, if it is surplus ammunition from certain countries such as the former Soviet Union, Yugoslavia or China, there is a good chance it uses corrosive primers, especially if it is manufactured before the 1980s. US-made ammunition has markings that can give a good clue as to whether the primers are corrosive or not. In case of doubt, there's an easy way to find this out. The user can take a cartridge, pull out the bullet from the front and empty out the powder from the cartridge and only leave the primer behind. Then. the user fires the empty cartridge onto a mild steel plate from a distance of about 1 inch from the muzzle, so that the primer chemicals are deposited on the plate. The user also fires another primer that is known to be non-corrosive onto another section of the steel plate. After firing the two cartridges, the user cleans the firearm as detailed in the procedure above, in case the suspect ammo is indeed corrosive. Then the user simply keeps the steel plate in a warm humid area for a few days. If the section where the suspect primer is fired over shows substantial rusting, then it uses corrosive chemicals.

Another way is to use bright common nails (which are nails made of mild steel with no coating) and pop a primers over one of these nails. If the nail rusts within a couple of days, then the primer is corrosive. The following video shows how this is done, using simple household tools:

Interestingly, the box of suspect ammunition actually says that it is non-corrosive on the box, but it turns out to be corrosive after all. Happy viewing!

What the heck is an "Arshin"?

Imagine you are now the proud owner of a classic Russian Mosin-Nagant M-91 rifle and are excited to try it out for the first time. So you take it out to the range, adjust the iron sights for a target 400 meters away and shoot at it. Upon shooting a few times, you examine the target and notice that you're not hitting where you're aiming at. Is there something wrong with the rifle? Actually, the answer may have to do with your misunderstanding about how the sight works.

A Konovalov type Mosin-Nagant adjustable iron sight.

In the above image, we see a Mosin-Nagant rifle sight called the Konovalov type. This sight acts as both a  tangent sight (for shorter ranges, marked as 4-12) and a ladder sight (for longer ranges, marked as 13-32). Note that the sight is sort of curved, as seen in the first image. Older Mosin Nagants have a flat shaped sight using the same idea.

The mistake that some people make is assume that these settings are in meters (e.g. 4 = 400 meters, 6 = 600 meters etc.) For Mosin-Nagants manufactured before 1930, this is not true -- in fact, these are calibrated in a unit called "Arshin" (plural: "Arshins" in English, "Arshiny" in Russian). So what the heck is an "Arshin" then?

To answer this question, we must go back to 16th century Russia, where this unit of length first originated. This unit was the Russian equivalent of the English "yard" measurement. Its actual length varied over the years, until Peter the Great standardized it in the 18th century to be about 27.95 inches long (or about 71.1 cm. or 0.78 yards). This continued to be how distances were measured in Russia until some time in 1925, when the Soviet Union officially adopted the metric system throughout the country. Konovalov type sights calibrated in arshins continued to be manufactured for some years after, until 1929.

Therefore, with older Mosin-Nagant rifles built before 1930, the sights were actually calibrated in arshins. Hence, when the slider is pushed to 4, the sights are set to aim at a distance of 400 arshins, not 400 meters. 400 arshins is approximately 285 meters, which explains why the rifle might not be shooting where the user expects it to!

Edit: The editor is indebted to Mr. Bernard Samartsev for his comments and corrections to the original article noted below.

The Greener Police Shotgun

The topic of today's study will be an unusual firearm, the Greener police shotgun.

The history of this unusual firearm dates back to shortly after World War I, when the British ruled over very large portions of the planet and the sun never set on the British empire. In several colonies around the globe, the British set up police forces employing locals to handle ordinary law and order issues, with British army garrisons to put down major rebellions and uprisings. There came a need to equip the local police forces of countries like Egypt, India, Hong Kong etc., with weapons to put down riots and jail escape attempts, without the need to involve the local army garrison. Therefore, the British government issued a specification for a new firearm for these police forces.

The requirements of this new firearm type were a bit unusual. First, many of these police forces were not well trained or educated, therefore the firearms had to be extremely simple, sturdy and reliable. They were expected to need minimal maintenance. Since there was a chance that the guns could be used against the British troops themselves, the guns had to be short-range weapons and single-shot only. Another request was that the cartridges used for these firearms should not be easily available. That way, if the police forces decided to rebel themselves, the British garrison could easily take them on.

Greener police shotgun. Click on image to enlarge.

With these requirements in mind, the firm of W.W. Greener, a well known manufacturer of firearms from Birmingham, UK, came up with the Greener police shotgun. The action chosen for this firearm was the Greener improved Martini action that we studied a couple of years ago. This reliable action was known for its simplicity and ruggedness. It was in the Martini-Henry rifle since 1871 and the Martini-Henry rifle was already used by many local militaries, which meant that many locals would be familiar with it. In addition, the barrel, springs and action were made of heavy-duty steel for extra strength and durability. The wood stock extended all the way to the end of the barrel, to prevent damage to the barrel. In addition, note the solid steel nose cap at the muzzle end of the barrel in the picture above. This cap served as extra protection for the end of the muzzle, so even if the gun was placed vertically with the muzzle-end on the floor, the steel cap kept the barrel about 1/4" off the floor. In addition, a bayonet could be attached to the steel cap. The butt-end of the stock also had a steel plate at the end, so it could be used as a club. The stock also had a compartment to store cleaning tools. The barrel of this shotgun had no rifling, so it could only be used as a short range weapon and since it used the Martini action, the user would have to manually unload and reload a new cartridge, each time he desired to fire it. The cartridge that this shotgun was designed to fire was a proprietary 14-gauge shell manufactured by Kynoch Ltd.

The original Mark-I model of this shotgun was released in 1921, mainly to colonial police forces in Egypt. However, it was soon discovered that unauthorized users could use a smaller commonly available 16-gauge cartridge in this gun and stuff the extra space with paper. In response to this, Greener released the Mark-III model shotgun, which had some improvements to prevent this:

New improved cartridge for the Greener Police Shotgun Mark III

In this newer model, the shotgun chamber was altered to take an unusual shaped cartridge. The base of the cartridge was the same diameter as a 12 gauge cartridge, but the front of it was narrowed down to 14 gauge. With an unusual bottle-necked cartridge shape like this, this cartridge could not be used with any other firearm.

In addition, the striker of the shotgun was also modified so that it could only be used with these unusual cartridges. Instead of a normal needle shaped striker, the new striker on this shotgun was shaped like a trident, with the outer two prongs longer than the middle prong. Note that the base of the new cartridge has a deep circular groove around the primer cap. The reason for this groove is so that the two outer prongs fit into the groove and the shorter middle prong can strike the primer of the cartridge. Therefore, the Mark-III shotgun could not use any other ammunition, except for this type of cartridge. If any other cartridge was used, the two longer outer prongs of the striker would strike the base of the cartridge first and prevent the shorter middle prong from striking the primer.

The end result of this was a cartridge that could not be used on any other firearm and a shotgun that could only fire a particular cartridge type. The British authorities were very careful to issue these cartridges in very limited numbers (about two or three per person). Therefore, if criminals stole these weapons or if the local government revolted, these guns would be useful only while the ammunition was available for them.

A lot of these shotguns were issued to colonial police forces in Egypt, Malaya, Hong Kong etc. Some of them were imported into the US in the 1930s, to be used in prisons. Greener continued to manufacture these shotguns even after British colonial rule ended in many parts of the world, until about 1975 or so. Used examples can be found on sale even today.

The Boys Anti-tank Rifle

In our last post, we studied the world's first anti-tank rifle, the Mauser T-Gewehr M1918, which was invented by the Germans. Today, we will study the history of another anti-tank rifle, the British Boys Anti-tank Rifle.

In the last post, we studied that the Mauser T-Gewehr was invented by the Germans because they didn't have any tanks of their own during World War I and had to come up with some solution to counter British and French tanks. Although tank armor had become thick enough to resist the Mauser by the end of World War I, the idea of an anti-tank rifle did not go away.

In 1934, the British decided to design a weapon that could penetrate light armor. The project was originally called Stanchion and was given to the Royal Small Arms Factory of Enfield, Middlesex, England to design. The chief designer of this project was a Captain H.C. Boys.

Captain Boys intended the rifle to be a bolt-action large bore rifle, capable of penetrating light armor, but light enough to be carried by a single person. He looked at the existing rifle cartridges of the day. The 30-06 and the .303 cartridges were found to be too weak (even the armor-piercing variants). The US made Browning .50 BMG cartridge was found to have some potential, because it could penetrate 3/4" armor at 100 yards, but they wanted something a bit more powerful than the .50 caliber cartridge. Therefore, they started with a Browning .50 caliber cartridge case, widened the case's neck to accept a .55 caliber steel-core bullet and added a belt around the cartridge base for proper headspacing (the idea of adding a belt on the base of a magnum cartridge was a British invention by Holland & Holland in 1910 for their larger hunting rifles firing the powerful .375 H&H magnum cartridges). This new cartridge weighed 946 grains (64 gm.) and had a velocity of 2495 feet/sec, delivering 17726 ft-lbf (24033 joules) of energy. By comparison, the .30-06 cartridge (all variants) delivers somewhere between 2800 to 3000 ft-lbf of energy. The job of manufacturing the new .55 caliber cartridge was given to Kynoch Ltd., a well known manufacturer of cartridges in England.

With the cartridge designed, the next goal was to design a rifle around this cartridge. It was simply a bolt-action design with a 5-round magazine, weighed about 35 lbs. (16 kg.) and was about 5 feet 2 inches (1.575 meters) long. The magazine was top-mounted and therefore, the sights were offset to the left to accommodate this. Tragically, the designer of this rifle, Captain Boys, died a few weeks before the rifle went into production in November 1937. Hence, the rifle was renamed as the Boys Anti-tank Rifle in his honor.

The Boys Anti-tank Rifle Mark I. Click on image to enlarge. Public Domain Image.

The rifle was manufactured in three variants. The Mark I variant (as shown in the image above) was the original design manufactured by the Royal Small Arms Factory in Enfield. Notice the round muzzle brake and the T-shaped bipod The Mark I* was given to the Canadian company, John Inglis & Co.  to manufacture and this variant had a V-shaped bipod and flat muzzle brake. The Mark II Airborne variant was designed to be carried by paratroopers and had a shorter muzzle.

When the Boys rifle was delivered, it was not effective against German panzers since the very beginning of World War II. While the bullet fired by this rifle could penetrate through mild steel plate of 23.2 mm. thickness (almost 1 inch thick) at about 90 meters (100 yards) range, the frontal armor of the German tanks was thicker and harder than this and therefore the rifle didn't fare very well against them. However, they proved to be more effective against armored cars and Soviet and Japanese light tanks. The Finns used them against Soviet tanks in 1940. The original steel-cored bullet was later replaced by a tungsten-core bullet (the .55 Mark II cartridge) in an attempt to improve its effectiveness. Despite the cushioned butt pad on the stock, the recoil from this weapon was terrific and often ended up bruising the operator's shoulder, which made soldiers very reluctant to fire it unless it was mounted to a support. This weapon did find some success with the British commandos of World War II (the Desert Rats), who mounted it to the top of their trucks and Bren carriers, when they went on raids in the desert.

Since the rifle had such a heavy recoil, an adapter kit was designed to be used by this rifle for training purposes.

What the adapter kit did was allow the user to mount a smaller .22 caliber rifle to the side of the Boys rifle and fire that via the trigger of the Boys rifle. This allowed for users to learn basic sighting techniques, without having them to bear the force of firing the Boys rifle during training sessions.

The Walt Disney Company was also commissioned to produce a training video for this gun during World War II:

Yes, that is a Disney cartoon animation at the beginning of the first video.

While the Boys rifle was largely ineffective in its original role of stopping German tanks, it was more effective against lighter skinned vehicles, early Soviet and Japanese tanks and armored vehicles. US Marines used it against Japanese bunkers as well. By the end of World War II, this rifle was seen as obsolete and was replaced by bazooka type anti-tank weapons. However, they were still used in the Korean war and in the Philippines until 1960 or so. One was even used in the 1960s by the IRA to disable a British Navy Patrol Boat HMS Brave Borderer by shooting out one of its turbine engines through the hull of the boat.