Does the carrier cycle back at all? I cannot tell if it is short stroking and not stripping a round, or not stroking at all. If it is short stroking, I would try a different recoil spring and guide. If it is no stroking, it is most likely a SEVERE head spacing problem, and i would not fire it. Examine the chamber- rough as hell or full of crud could hold the round fast long enough to lock the action, but this is the least likely cause.