Buddy had a fortunate mishap today

Buddy of mine was testing a new buffer for reliable cycling on some sub 300blk when he had the round detonate inside the chamber, and I’m assuming the bullet was sent. We don’t think it was not a squib but I could be wrong, my hypothesis is that it was a double charged round. A guy he knows gave him 500 of these through a trade, other guy has had zero issues but like I said to my buddy, it only takes one. They are reloads, fairly sure. Other reasoning could be the case was weak? He had to pry the bolt out of the upper and I’m not sure if that ripped the rim off or if it was already separated from the detonation. He’s okay, just a little dazed from what he said sounded like a cannon.