When I have seen brass fail in this manner (mabe a dozen times while in the marine Corps), it was always the result of a barrel obstruction--namely a round lodged in the bore--the result of a squib load, contaminated powder, no powder, etc. In the Service, you learn your malfunction drill so well that if in the middle of a rapid fire magazine you pull the trigger and nothing happens, you instinctively pull back on the charging handle and chamber another round and pull the trigger again, zip, zip, zip in about a second, never thinking about why the action had just failed to function normally. Then you drop the hammer onthe round you just chambered and kaboom!
Interestingly, this most often occured with L110 Tracer Ammo which had been spread-loaded with ball ammo in magazines. Had similar events with 7.62 linked ammo with ball rounds marked say WCC 71 linked 4:1 with tracers marked REM 62. This was the result of us (USMC) buying old lots of tracer ammo packed in 20-round boxes intended for the M14 Rifle from the Army "on the cheap", then inserting them into an ammo line making belted ammo for the M60.
A squib round may lodge anywhere in the bore from just outside the chamber (worst case) to down towards the muzzle (not so bad but still catastropic). This is nothing new or unique to the AR.
I remember in the olden days the FBI's spec for their revolver barels required S&W to use a bore design that if a squb load lodged a bullet in the bore but did not lock-up the cylinder (i.e., somewhere in the rifling), then the shooter could "safely" fire a second round in an emergebcy situation.
Bottom line is the AR chamber does not fully support the case head's web so as to help in reliable feeding from the magazine. So when you pop the cap on 50,000 pounds of presure something is going to give--and that is the area of brass not supported in the chamber.
ColdBlue sends...
(CB is David A. Lutz, Lt. Col. USMC (Ret'd) (1968-1991)
Former (now retired) VP MilOps @ Knight's Armament Company (KAC) (1994-2012)
"...if you can read this, thank a Teacher,
if you are reading this in English, thank a Veteran..."
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