so a friend of mine came up for me to swap out the FCG in his AR for him. While I’m working at the bench, he’s looking around the gun room. In the process he manages to grab one of my loaded pmags of 300 blk and is walking around with it. I get his firearm done and tell him we can step out to the range here at my house to try it out. He mistakenly grabs the pmag he’d been walking around with instead of his off brand mag full of 223. Of course I never know he’s even touched a mag full of 300, muchless that he had one in his pocket. He sits down at the bench, inserts mag, chambers a round, no problems at all up to this point. I’m standing behind him watching the target when kaboom, and smoke rolls from around him and his gun. I immediately knew something was wrong without even seeing the firearm. Once we pulled the mag out I explained to him what had happened (in a cuss your friend for being a dummy kind of way) as soon as I seen what had happened. Very lucky there were zero injuries. We had a little lesson session following this. Here are some pics.