Title says it all. Specifics of brand, etc is unimportant at this time. Presume that ammo, mags, and gun are in order.
Some things that will help...
Bolt locks to the rear on an empty mag, mag hardware is practically new.
Carrier is staked properly via MOACKS.
No gas leakage around gas block.
Bolt recedes in to carrier when set upright on a table. (Gas ring test?)
No insert on extractor spring, barrel is sub 14.5".

