It think it was the slide lock spring. There is a sequence on when it is to be installed and where with the trigger assembly and I either did it before or after I should have that messed things up. Somehow the son-in-law figured it out, as he had done the same thing on one. I've forgotten which order since it has been a while. I could manually rack it and everything was fine but shooting it, it would sometimes stovepipe or lock back early, if not mistaken. I remember it was very frustrating until it was fixed. It has been fixed for a while. I'll see if I can email him. He probably remembers.