Because sometimes the wrong caliber will chamber. Results, FOOM.
Well, that’s why you pay us the big bucks. This is not a mild AR kB! that traces back to an out of battery fire or one lousy handload. In this case, 100% of the round’s energy came back through the firing pin hole in the bolt face. And shattered the bolt and carrier. What could make an AR do that?
An obstructed barrel right at the chamber. Period. Sure, there’s a vanishingly small possibility that you got a rare combination of a round loaded with pistol powder with the previous round being a powderless squib, combined with a tight bore and wide bullet so that it didn’t go the usual 7-8″ into the barrrel. But the 95% probabilty is: .300 Blackout round in a 5.56 rifle.
He doesn’t know what happened, our fourth point of contact. He claims it was ordinary M193 5.56. Hmmm. Maybe the other rounds in the mag were, but we’re betting there’s a .300 supersonic slug lodged in there.
Read the Remainder at Weaponsman