Posted by Nathan Schueth on May 12th 2016
Why 9310 Steel in an AR-15 / M16 Bolt?
"9310 is not MIL-SPEC, it's better than MIL-SPEC"
Last week Faxon announced the release of their GUNNER series of Modern Sporting Rifle bolt carrier groups. Its been a pleasure working on the carriers and we are thrilled to have them released to our customers to rave reviews. We were able to create something never done before: A steel carrier, keeping forward assists, full-auto compatible, and less than 6.5 ounces!
One of the questions we have been getting is why we opt to not follow the "Mil-Spec" for our bolts, specifically why we are not using Carpenter 158 steel for the bolts?
The answer is simple: "Mil-Spec" is just the minimum requirement and metallurgical technology has caught up to design of the AR-15 since its creation.
The reason for Carpenter 158 as the "Mil-Spec" is rooted in history when the AR-15, chambered for the lower pressure .223 Remington round was upgraded to the higher pressure 5.56 NATO loads. The original material selection for the bolt was found to have inadequate service life. The military went searching for better material, constrained by the size of the bolt. Carpenter Technologies' 158 blend of steel was selected as it provided adequate strength and wear characteristics. It has been the "Mil-Spec" since.
Since the creation of Carpenter 158, the metals industry has continued to innovate, creating new blends of steels. 9310 is a AISI standard grade of tool steel that makes it about ~7% stronger than "mil-spec" Carpenter 158 steels, when appropriately treated. Its commonly used in the aerospace industry where toughness, impact strength, and solid wear characteristics are required.
From the base material, we took what we liked from the "Mil-Spec" and continued development. Our 9310 bolts are still magnetic particle inspected (ensures consistent material), shot-peened (surface strength), and heat-treated (for shock, abrasion, and wear) like the Mil-Spec. With 9310, the heat-treat is critical and all steps are completed with full certifications.
From there, Faxon opted for nitrided bolts instead of manganese phosphate finihs. "Nitriding" or nitrocarburization is applied for us via Quench, Polish, Quench method. The process imbunes nitrogen into the steel hardening the surface, reduces friction, and increases corrosion resistance. We use the same process on all of our barrels. This creates a slicker, more corrosion resistant bolt that is simulatneoulsy harder to resist wear.
As such, the combination of 9310, correct treatment methods, and QPQ create a better bolt; one that exceeds "Mil-Spec".