So you're saying the gun won't cycle. It will strip a round out of the magazine, fire, and eject but won't load the next round? It will shoot a single round, eject, but not lock back? If that is the case, then it is the gas system. One of the things not mentioned is to make sure the bolt key ois staked properly and not loose. This site is broken, so I can't post a picture, but it the part of the bolt that the gas tube mates with. As mentioned above make sure that the gas block is lined up with the gas hole in the barrel and the gas tube is lined up in the gas block. You can check this by plugging the muzzle and blowing air through the barrel.