I had a similar problem with my LMT SBR upper when I first got it and it gave me fits. When I finally broke down and called LMT the first question out of the tech's mouth was what weight buffer am I running? Well, an H2 of course...everyone knows you should run an H2 in a Carbine length gas system right? Nope. LMT drills their gas ports to cycle with a standard weight carbine buffer. So I replaced the H2 with a carbine weight buffer and the stupid thing has run like a champ ever since. Never skipped a beat.
When gas keys are staked and things aren't perfect its not unusual to break off the bolt close to the carrier or just under the bolt head. BCM bolt carrier groups are made by fallible people using imperfect materials. Like all mechanical parts they have a failure rate...
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Last edited by Big Bird; 11-15-2012 at 21:01..