You might also check the condition of your gas rings, if you hold bolt carrier upside down with bolt key etc. removed, bolt should not fall out by itself (do this without lube).If ok. Clean buffer tube,lube , reassemble , go to range.Fire a few rounds to blow lube out of rings, Load one round in mag, Fire and see if bolt locks back, if it does not you may have carbon buildup in gas tube or front sight assy.Also check carrier key (the thing on top of bolt the gas tube goes in) for tightness. Travis F.