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.