8620 vs. 9310 vs. Carpenter 158 vs. S7 Tool Steel
What do all of these numbers have in common? They are alloys typically used with a bolt carrier group. Which alloy is best for your bolt carrier group? First, let’s first discuss a little background on the design of the original AR-15 gas system.
The original AR-15 designed by Jim Sullivan (not Eugene Stoner) was designed to work with ammunition having a chamber pressure of 50,000 pounds per square inch (psi), a 20-inch barrel and a rifle–length gas system. When the military resurrected the carbine program in 1985, they told Colt they wanted a 14½-inch barrel for use on the new carbine. However, this new carbine exceeded the design of the AR-15. The rifle Sullivan designed had 14,500 psi in the gas tube while the carbine-length system had 19,000 psi. Pressure inside the bolt carrier went from 1,500 to 2,500 psi. The significant boost in gas system pressure means that the bolt has to unlock with much more binding force on the lugs. Binding force is the friction between the bolt’s lugs and the lug abutments in the barrel extension. Too much binding force will cause bolt lugs to shear off. When an AR-15 experiences a catastrophic failure, it is almost always the bolt that breaks. It’s either a lug on either side of the extractor or the bolt body where the cam pin passes through it. This is why any AR-15 should always use the longest gas system possible for the chosen barrel length.
Thus, any short barrel rifle needs to have some type of enhanced bolt to handle this increased force. “Enhanced” is more than just putting a high–speed, low–drag coating on a bolt carrier or bolt. That does not qualify as an enhancement; that’s a cosmetic touch.
When it comes to the strength of an alloy, there are two measurements common in the context of a bolt carrier group: tensile strength and yield strength. Tensile strength is the measure of the maximum stress that an object can withstand without being elongated, stretched or pulled. Yield strength is the force exerted at which the item permanently deforms.
Reminder: MIL-SPEC doesn’t mean greatest ever, or worst, it’s just in spec to a military contract. Although, newer designs, coatings, and research suggests most MIL-SPEC components are inferior to other available options.
8620 (normalized). This is the MIL-SPEC (MIL-S-16794) alloy used to machine the bolt carrier.
Tensile Strength: 640 MPa
Yield Strength: 360 MPa
Tensile Strength: 910 MPa
Yield Strength: 570 MPa
Carpenter 158. This is the MIL-SPEC alloy used for the bolt.
Tensile Strength: 1103 MPa
Yield Strength: 931 MPa
Tensile Strength: 2030 MPa
Yield Strength: 1520 MPa
With the exception of Carpenter 158, the above numbers are generalizations that can be deviated if the alloy is not normalized or hardened according to specs. For example, the Sharps Rifle Company Relia-Bolt is machined from S7 tool steel, but they publish tensile strengths of 1550-1600 MPa (220,000-235,000 psi). While this remains significantly stronger than Carpenter 158, it’s a ~24% reduction in strength when compared to the general tensile strength noted above. While some manufacturers decline to publish their proprietary alloy used (e.g. BCM declines to publish the alloy used for the bolt carrier; LMT declines to publish the material used for their bolt, but it is reportedly much stronger than Carpenter 158), they should at least be candid regarding the strength of their alloys when catastrophic failure can occur as a result of using an inferior alloy.