Grendel Kaboom w/ Young's MFG Bolt

[LRRPF52]

I had a 5.56 bolt fail at the cam pin hole, I caught it before it let go. It looked nothing like that bolt. My guess without seeing test results is embrittlement from to long of a hardening process.

The 5.56 bolt I had fail was a BCM, so even top shelf companies can have failures.

Especially when you HPT a bolt, you're asking for it to break earlier than it should.

Some of the best companies in the AR15 and AR10/SR25 world refuse to HPT bolts for that reason, like Diemaco, KAC, etc.

I cracked 5.56 bolts at the cam pin hole as well.

This problem is metallurgy though.

A little history of bolts (5.56)
The US Army and USAF did a joint study in the early 1960s after the first production models of AR15s had been fielded and started to accumulate a round count on them. They were seeing bolt failures enough to initiate fleet testing, and found that hard chrome plating plus inadequate 8620 alloy were contributing to broken bolts.

The conclusions of that study showed most rifles cracked at the cam pin hole, and the others sheared the lugs adjacent to the extractor. Their solution was to use a 9310 alloy even back then, and drop the hard chrome plating of bolts due to hydrogen embrittlement tendencies of the process. This is why the Colt 601 (Green rifle M16) and Colt 602 had chromed bolts, which were later replaced with phosphated bolts. Phosphating is basically micro-fracturing of the surface, so not the best coating or surface treatment either.

At the end of the day, the main focus of the AR15 was to be a lightweight rifle. The stress-bearing components were scaled down from the AR10 for a different cartridge with lower chamber pressure (.222 Remington), and then pressure was increased with the .222 Remington Special (renamed the 5.56x45 and type-classified as M193), followed by a propellant change without consultation or notification of the designers shortly after that. Bolt thrust became an issue for guns shot in volume with the early chromed bolts.

These issues were solved by the time the Colt 603/M16A1 was standardized, since better alloy and processed were used, as well as the taming of the cyclic rate with a heavier buffer. You would rarely see broken bolts in 20" RLGS M16s, M16A1s, or M16A2s. Once the carbines became the focus for production, broken bolts started occurring more frequently, primarily in SOCOM since they actually train with tons of live fire. The Canadians said they scratched their heads when they saw so many reports of broken bolts, since they weren't seeing it in their guns and clients, who shoot just as much (SAS, SBS). Their bolt metallurgy processes are different than the US TDP/Mil-Std, with the absence of HPT being the most notable difference, tighter controls on heat treating and dimensions being the others.