Metals Used in Firearms - III

In our last couple of posts, we looked at certain types of steel alloys which are used in firearm construction. In today's post, we will look at another type of steel alloy that was invented in 1912 and used in some firearms: stainless steel.

Stainless steel is a steel alloy that contains a high percentage of chromium (greater than 10.5% by weight). Unlike ordinary carbon steels, it has good resistance against corrosion and rusting. This is because of the high chromium content. What happens is that the chromium at the surface of the object reacts with the oxygen in the air, to form a thin layer of chromium oxide. This chromium oxide layer prevents oxygen from reaching the inner steel and therefore blocks rusting and corrosion. It must be remembered that while stainless steel is rust-resistant, it is not rust-proof.

Fittingly, the invention of stainless steel was actually related to firearms. Harry Brearley, an English chemist was working in Sheffield, England for Brown Firth research labs in 1912, trying to find a new steel that could resist erosion caused by high temperatures of gun barrels. It was already known at that time that adding a little chromium to steel increases the melting point of steel. He was trying to establish precisely, the relationship between melting points and chromium content of various steel samples. As part of this study, he was required to study the microstructure of the various steel alloy samples and to do this, he had to polish and etch the samples first. The standard way to do this was to use a weak solution of nitric acid and alcohol to do the etching, but as Mr. Brearley found, some of the samples were exceptionally resistant to these chemicals. After a bit of investigation, he determined that the high chromium content of these samples was responsible for the exceptional resistance to acid. From this research, a whole new industry of manufacturing stainless steels sprung up around the Sheffield area.

Like chrome-moly steels, there are also different grades of stainless steels and only some grades are used in the manufacture of firearms. For instance, SAE grades 410 and 416 are used for firearms barrels. They are both steel alloys with high chromium content (11.5 - 13.5% for 410 stainless steel and 12-14% for 416 stainless steel). The main difference is that 416 stainless steel contains a bit more sulfur in it, which makes it easier to machine than 410 stainless steel, which makes the barrels cheaper to produce. However, 410 stainless steel retains its toughness better and performs better in freezing conditions. Some companies make custom alloys, such as Crucible Specialty Metals' 416R, which is specially designed for precision steel barrels. Another stainless steel alloy used by some makers is 17-4 PH (PH standing for Precipitation Hardening).

Some of the other parts of the guns are also made of 400 or 300 series of stainless steels. The 300 series is more resistant to corrosion than the 400 series of steels, but cannot be hardened as easily, so it is used for parts that aren't exposed to huge forces.

The advantage of stainless steel alloys over chrome-moly steel alloys is that they are easier to machine and resist heat erosion better. However, they are a bit more expensive and cannot be blued using conventional methods. The US military prefers chrome-moly barrels, but most competitive target shooters prefer stainless steel barrels, because they can be machined more precisely and keep their accuracy longer. This is why the majority of match-grade barrels are made of stainless steel.

Metals Used in Firearms - II

In our last post, we looked at steel alloys commonly used in modern barrels. In today's post, we will continue to look at some more steel alloys used in parts of a modern firearm.

Besides the 4140 and 4150 steels that we looked at in the previous post, some other variants are also used for manufacturing barrels. For instance, adding a small amount of the element vanadium to steel (less than 1%) increases the strength, toughness and heat resistance of steel. An alloy called 41V45 (which is basically a steel with similar elements of 4140 or 4150 steel, but with only 0.45 % carbon content and a little vanadium added) is often used with barrels made using the hammer forging method. 4340 and 4350 steels are also used by some barrel makers, but these steels are more difficult to heat treat properly and therefore the cost of the barrels increases. However, if properly manufactured, a 4340 barrel can be much stronger than a barrel made of 4140 steel.

While the barrel of a firearm has to resist large pressures, it is not the only part of the firearm that is subject to such large forces. Another part that also receives a lot of force is the bolt of a firearm. In the case of the M16 family, the US military selected a steel called Carpenter 158 (also known as "C158" or "Car 158") to be used for manufacturing the bolt. Therefore, any AR15 or M16 bolts that claim to follow military specifications (or "mil-spec" for short) should be made of C158 steel as one of the requirements.

The interesting thing about Carpenter 158 is that it is a proprietary steel alloy and its formula and method of manufacturing are not publicly defined. Therefore, there is no SAE standard for it and the sole manufacturer of this steel is a company called Carpenter Technology in Pennsylvania. They do not manufacture this steel continuously and only do a certain number of mill runs per year, which means it is not always available. They also only sell this steel in large amounts and require the customer to buy a lot of it at a time. Therefore, only large companies like Colt, Fabrique Nationale (FN), Daniel Defense etc. can afford to buy this steel.

However, C158 isn't the only steel alloy used to make bolts. Bear in mind that the US military selected C158 in the 1960s and there have been other (and sometimes better) steel alloys developed since then. For instance, some manufacturers use 9310 or 8620 steels to make bolts. The advantage of these steels is that their chemical compositions and manufacturing methods are publicly specified by SAE (Society of Automotive Engineers - the organization responsible for specifying standards for different grades of steel in the US) and there are many manufacturers of these steels. Therefore, supply is less of a problem. Also, 9310 and 8620 steels can be purchased in small quantities and are therefore suitable for smaller manufacturers who cannot afford to buy large amounts of C158 steel at one time. If machined and heat treated properly afterwards, bolts made of 9310 steel can be even better than C158 steel. Another steel alloy used for bolts by some manufacturers is Aermet 100 (which is also used in the landing gear of jet fighters aboard US aircraft carriers). This is actually another proprietary steel alloy made by Carpenter Technology and is reputed to be far superior to C158 and 9310 steels. However, it requires a double hardening treatment process after machining to reach its full potential. In general, bolts made of 9310 steel are the cheapest to produce, followed by bolts made of C158 steel, and bolts made of Aermet steel are the most expensive. However, bolts for AR15/M16 made of 9310 or Aermet 100 cannot be called "mil-spec" even if they exceed the military standard specifications, since the US military specification says that only C158 steel should be used for the bolt of a M16.

The above mentioned 8620 steel is also used by manufacturers to make bolts for certain rifle models. For instance, it was extensively used in World War II for the M1 Garand rifle. Actually, the original M16 rifles also used bolts made of 8620 steel, but the US military found that the bolts were wearing out after 40,000 - 50,000 shots, which is why they went with C158 steel for the bolt. Still, 8620 steel is used for other parts, for instance, the bolt carrier and the receiver. This is because this steel is suitable for casting, welds very well, has very good machining properties and can be heat-treated to become tough and strong. The M1 Garand and the M14 both use 8620 steel for receivers.

For smaller parts that are not subject to much stress, commonly available carbon steel alloys such as 1020 steel are used. This is used to make small parts, such as the trigger guard, the sights, the rifle sling swivels, smaller pins and screws etc. 1020 steel is commonly available, cheap, easy to weld, flatten, machine, forge and heat treat and can be used for any applications where core strength is not critical.

In our next post, we will look into stainless steel grades used in firearms.

Metals Used in Firearms - I

Over the years, we've briefly discussed the properties of some of the metals used to construct a firearm (such as here and here). In today's post, we will revisit the topic of metals used in firearm construction in greater detail.

In today's post, we will skip over metals used from a bygone era (e.g.) brass, iron, bronze, gunmetal etc. and restrict ourselves to metals that are used in modern firearms. The main metals and alloys used are: steel, stainless steel and aluminum. Of course, there are different grades of these, such as AISI 4140, AISI 4150 etc. and we will study what all this means in today's post.

Briefly speaking, a metal used for gun barrels should be capable of handling large stresses, because it will experience large amounts of pressure (over 50,000 pounds per square inch or 340,000 kpa for metric speakers). It should be strong and elastic and ideally, it should be easy to machine and somewhat cheap. With that said, let's look at the first of the metal alloys: steel.

Steel is an alloy of iron mixed with other elements. The most important of these "other elements" is carbon. Pure iron is actually a soft metal and the addition of carbon allows the steel to be hardened much more than iron. However, an excess of carbon in the steel makes the steel brittle, so the quantity of it has to be carefully controlled. As we have studied previously, steel consists of crystals and it can exist with different crystalline structures. The shapes of these crystalline structures control the physical properties of the steel (such as hardness, elasticity, melting point etc.), The different crystal structures are sometimes called "phases" and there are several of these, such as ferritic phase, austenitic phase, martensitic phase, ledeburite phase, pearlite phase, cementite phase etc.

Steel Phase Diagram

Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported License by Christopher Dang Ngoc Chan.

Steel can be switched from one phase to another, by heating, adding or removing other elements and controlling the cooling rate. The diagram above shows different steel phases and how it changes from one phase to another one, based on the temperature and carbon content. However, carbon isn't the only element added to iron to make steel, there are also other elements added, which also help to change the properties of steel. For instance, adding nickel and manganese makes steel more elastic, vanadium adds hardness, chromium adds hardness, increases melting temperature and adds corrosion resistance. Adding tungsten keeps the steel from forming cementite and forming martensite instead.  Sulfur, nitrogen and phosphorus make the steel more brittle, so these are removed during steel manufacture etc.

In America, the Society of Automotive Engineers (SAE) is responsible for maintaining standards for different grades of steel. Some of these specifications were originally developed by the American Iron and Steel Institute (AISI), but since SAE and AISI were often developing standards for the same materials, they decided to combine their efforts and AISI has turned over maintenance of standards to SAE since 1995. In other countries, there are similar organizations, such as the British Standards Institution (BSI), European Committee for Standardization (EN), Japanese Industrial Standards (JIS), German standards (DIN) etc.

Per the SAE standards, the steel grades are labelled with a four-digit number (such as 1060, 4140, 4150 etc.). The first digit indicates the main alloying element of the steel. For instance 1xxx is carbon steel, 2xxx is Nickel steel, 4xxx is molybdenum steel, 7xxx is tungsten steel etc. The second digit indicates the secondary alloying element(s) in the steel and the last two digits indicate the amount of carbon in hundredths of a percent by weight. For instance, 1060 steel is a steel alloy that only contains carbon and has 0.60% by weight of carbon it it. Similarly, 4140 steel has molybdenum and chromium in it, with about 0.40% by weight of carbon and 4150 steel has molybdenum, chromium and about 0.50% by weight of carbon in it. In reality, there is a little leeway allowed. For instance, according to SAE, the allowed percentages by weight for 4140 steel are: Chromium: 0.8 - 1.1 %, Manganese: 0.75 - 1.0 %, Carbon: 0.380 - 0.430 %, Silicon: 0.15 - 0.30 %, Molybdenum: 0.15 - 0.25 %, Sulfur: up to 0.040 % allowed, Phosphorus: up to 0.035% allowed, Iron: 96.785 - 97.77 %. The standards for 4150 steel are similar to 4140, except that the carbon content allowed is 0.48 - 0.53 % by weight and the iron content is correspondingly reduced to 96.745 - 97.67 %, with all the other elements in the same proportions as 4140 steel.

Other countries have similar standards for steel grades. For instance, in Europe, the EN standard 42CrMo4 steel is about the same specifications as SAE 4140 steel, as are the British standard EN 19, Japanese standard SCM 440, German standard 42CrMo4 etc.

Now why did we mention 4140 and 4150 steels? That's because these are steel grades that are heavily used in the firearms industries to make barrels (they are also used to make gears, axles, connecting rods etc. by the automotive industry). These steels belong to the "chrome-molybdenum" or "chrome-moly" family. While these alloys do contain chromium, it is not as much as the chromium content found in stainless steel and therefore, they have less corrosion resistance compared to stainless steel. However, chrome-moly steels can be surface hardened, where the interior of the piece retains its properties, but the surface is hardened against wear and tear.

The standards for 4140 and 4150 grade steels has been around since about 1920 or so. As we noted a few paragraphs above, the difference between 4140 steel and 4150 steel is the carbon content (about 0.40% for 4140 steel and 0.50% for 4150 steel). So the difference between these two steel alloys is only that 4150 steel has approximately 0.1% more carbon. However, this extra 0.1% makes a big difference in the hardness, heat resistance and resistance to wear of 4150 steel, compared to 4140 steel. It also makes 4150 steel so much harder to machine and therefore increases the cost of manufacturing barrels.

The US military wants their barrels to last longer and work well under automatic fire, therefore they are willing to pay the extra costs associated with 4150 grade steel barrels. When you see barrels labelled "mil-spec", these are likely made of 4150 grade steel. That does not mean 4140 grade steel is bad -- in fact, it works well for civilian applications and does last for a long time as well, which is why you find so many manufacturers making barrels out of 4140 steel.

In the next few posts, we will study other metal alloys used in firearms.

Steam Chests

A few posts ago, when we studied heavy machine guns, it was mentioned that one of the features of some of these machine guns is a jacket filled with water, which surrounds the barrel and helps to prevent it from overheating.

Of course, if several rounds were to be fired rapidly, the intense heat of the barrel would cause the water to turn into steam and evaporate, thereby reducing the cooling effectiveness of the jacket. In the early Maxim heavy machine guns, the users would simply unscrew a cap on the top of the water jacket and refill it with more water, whenever the water level in the jacket got low.

A Maxim machine gun. Click on the image to enlarge.

Image licensed under the Creative Commons Attribution-Share Alike 3.0 License by Jonathan Cardy at Wikipedia

However, this presented a logistical problem to its users because then they needed to position the heavy machine gun next to a supply of water and this was not always possible in the battlefield. Resourceful users found that they could urinate into the water jacket in an emergency, but it is not possible to do that on demand, therefore it is preferable that a reliable supply of water be nearby. Another problem was that the steam rising from the barrel could also give away the position of the machine gun. This was a problem in trenches and rough terrain, since heavy machine guns cannot be moved as quickly by troops without a vehicle. Another issue was that if the steam was not allowed to escape out of the water jacket, the steam pressure inside could build up to the point of rupturing the jacket.

To counter these issues, newer models of heavy machine guns at the end of World War I were issued with steam chests. A steam chest, or more properly, a steam condensing chest, is simply a container that is placed below the barrel and is connected to the water jacket via a short pipe (or pipes). When steam is created by the hot barrel, instead of allowing the steam to escape out of the water jacket, it is transported through the pipe into the steam chest, where it condenses back into water. Periodically, this water is poured back into the water jacket for reuse. This system allows users to conserve their water supply for much longer and also prevents steam from rising and revealing their position to enemies.

A Browing M1917A1 machine gun. Note the steam chest attached to the front of the water jacket.

In the above image, we see a Browning M1917A1 machine gun. The steam condensing chest is the rectangular can at the front with the rubber hose coming out of it. The steam coming out of the water jacket flows through the rubber tube (which has some water in it as well) and into the rectangular can, where it cools down enough to become water again. Periodically, the crew lifts the can above the height of the water jacket, which causes the water in the can to flow back through the tube into the water jacket.

In the above image, note that the can is a custom made device with its own folding carrying handle. However, any convenient container could be used as well, as the image below shows:

A British Vickers machine gun with its condensing chest. Click on the image to enlarge. Public domain image.

In the above example, we see a Vickers machine gun, as used by the British military. The steam chest on the right of the picture has some faint lettering on it, which may be discerned by clicking on the image to enlarge it. Still having trouble reading the letters?? The image below shows a closeup of the steam chest with the letters clearly visible:

A can of Shell gasoline! Public domain image. Click on the image to enlarge.

Yes, that is a can originally meant to hold gasoline (or petrol, for non-American readers), made by Shell, the well known oil company. These cans were issued to British troops as standard equipment with Vickers machine guns, to be used as steam chests!

In more advanced models, the steam chest was equipped with a manual pump, which allowed the users to pump the water back into the water jacket without moving the steam chest.

A Colt MG 38 machine gun. Click on the image to enlarge.

In the above image, we see a Colt MG 38 machine gun with its steam chest positioned at the left of the image. Note that there are two pipes coming out of the water jacket into the steam chest. The pipe on the right of the picture transports the steam out of the water jacket into the steam chest and the other pipe sends water back from the chest into the water jacket. Also note that the steam chest has a handle to the left of it. The user turns this handle, which operates the manual pump inside the steam chest and sends water back into the water jacket. With this design, the user does not need to lift the can to pour the water back.

It must be noted that steam chest systems aren't air-tight, so a little steam does always escape out. However, they allow their crews to re-use their water for a lot longer.

Steam chests were supplied with most heavy machine gun models made in between World Wars I and II. As water-cooled heavy machine guns began to be phased out of military inventories and replaced with air-cooled models, the need for steam chests went away as well. With the invention of improved metallurgical techniques for making barrels last longer at higher temperatures, as well as the development of quick-change barrels, the extra weight of the water jacket, water and the steam chest were simply not worth the trouble for most users. That is why no modern machine guns use steam chests any more.

General Purpose Machine Guns

In our last couple of posts, we looked at medium machine guns. In today's post, we will look at some developments in the General Purpose Machine Gun (GPMG) category (otherwise called Universal Machine Gun or UMG).

A general purpose machine gun is a weapon designed to use both magazines and belt feed, firing full sized rifle-cartridges, which is air-cooled and designed to be used either as an infantry support weapon (i.e. like a light machine gun), or as a vehicle mounted weapon (i.e. like a medium machine gun). Since it is air-cooled, a GPMG usually features quick-change barrels, so it can continue to fire on automatic mode for longer periods of time. A general purpose machine gun can be mounted on a bipod or a tripod, or even from a vehicle, such as a jeep or a helicopter.

The first GPMGs were something we'd just studied in the previous post. The MG 34 was the first general purpose machine gun. Carried by an infantry man with a drum magazine and mounted on a bipod, it served as a great infantry support weapon used for offensive operations. By switching the magazine with an ammunition belt and mounting it on a tripod, it became a very good medium machine gun. Mounted on a tank or a vehicle, it was an effective anti-aircraft and defensive machine gun.

Public domain image of a MG 34 machine gun. Click on the image to enlarge

The only problem with the MG 34 was that it was somewhat harder to manufacture, due to the fact that it needed some precision machining. Due to this, the Germans came out with the MG-42, which was easier to manufacture, more reliable, and as an extra bonus, had a higher rate of fire as well!

A MG-42 mouinted on a tripod. Click on the image to enlarge. Public domain image.

The MG-42 design was extremely successful and some variants (such as the MG3 and MG74 models) are still in service in some militaries around the world. It also influenced other countries to manufacture their own GPMGs based somewhat on the MG-42 design.

The US military was one of the first to pick up the idea of a GPMG from the Germans and started working on a design in the late 1940s. One of the requirements was that this gun had to be chambered to fire 7.62x51 mm. NATO ammunition and another requirement was that it should be capable of being fired accurately from the shoulder as well. After a number of trials, the final design was approved in 1957 and was called the M60 machine gun. The M60 served in various branches of the US military during the Vietnam war. It was carried by infantry units as a Squad Automatic Weapon (SAW) to support infantry operations and was also mounted on river patrol boats (PBRs), M113 Armored Personnel Carriers (APC) and as a door gun on helicopters.

US Marines with a M60 in Vietnam. Public domain image.

While the M60 saw service in Vietnam, some design flaws became obvious. For one, it had some jamming issues in muddy and humid conditions. It did better when used in static-defense or helicopter mounted roles, because it could be stored in cleaner conditions and regularly maintained. One more problem was the design of the barrel, which had a permanently attached bipod and this made barrel changes more difficult and took longer to accomplish. While some improvements to the design were made, the US military rejected the M60E3 and went with the Belgian FN MAG (which we will study in a second) and designated it as the M240 in US service.

Around the time that the US was developing the M60, the Fabrique Nationale company (FN) of Belgium was also developing their own GPMG, which they called the FN MAG (the letters MAG stood for Mitralleuse d'Appui Generale, which is French for "General Purpose Machine Gun"). Like the M60, the FN MAG is also designed to fire 7.62x51 mm. NATO cartridges. While it is heavier than the M60 and uses a more complicated gas operated system, it is more reliable compared to the M60. This is why it was also adopted by the US military as the M240 to replace the M60s in service. The M240 was first mounted on to tanks in 1977 and later adopted by other US military branches during the 1980s and 1990s.

US Marines firing a M240G mounted on a tripod. Click on the image to enlarge. Public domain image.

The FN MAG and the M240 continue to be used in service in many military forces around the world.

Other GPMGs include the Soviet PK series, the Heckler & Koch HK 21 etc.