I don't know if the gun slipped or he wasn't totally focused but he had the barrel off and the bolt back. All of a sudden you hear a loud thwack! And a gas piston bouncing on the floor. Somehow the bolt release button got pressed and the bolt slammed forward. You could see how sick to his stomach he felt. We looked the gun over and everything looked to be all right other than a small indentation where the bolt smacked the receiver. Anyone else ever had the misfortune of seeing this or even doing it? I know the instruction manuals say it is a bad thing and could damage the receiver. What exactly could happen? His gun looked okay and hopefully it is so he can sleep at night.