Load 1 round in the mag, chamber it, and fire. If the bolt locks back each time, you're getting plenty of bolt travel. Then load 2 rounds, fire round 1, and see if the second chambers. If this doesn't work, check you magazine. That's the extent of the detective work I've encountered on the AR. Well, other than the strange variation that was so over gassed, it was feeding two rounds per cycle.
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