I was at the range shooting a Garand when an older gentleman started shooting his Rock River AR-15. He fired for a while then ran into a problem. The rifle would chamber a round but it would not fire. I looked at a couple of the rounds and could see a light firing pin mark on the primer but no more of a mark then should be expected when a round is chambered. I opened the rifle up and pulled the bolt/bolt carriage out and found that the buffer retainer and its spring had come loose and had been “run over” by the bolt. I don’t even know what is supposed to hold the retainer in place so I was wondering how one would come out like that. Any thoughts?