It looked like he had a round pop a primer but not ignite the powder on the last round of the first mag. You'll notice that he extracts a case when he takes the mag out. Then when he puts a new mag in he has to hit the forward assist, that doesn't work so he chambers another round and uses the forward assist, and so on until he beat the stuck bullet in far enough for a live round to chamber. The primer has enough force to send a bullet into the rifling sometimes enough so that a new cartridge can be chambered. When he fired the next round with the plugged barrel the gun blew up. He should of checked out why the last round in the first mag didn't fire, or at least checked to see why rounds stoped chambering. I've had the powder not ignite in a mini14 with pmc ammo, and a S&W 5946 with winchester ammo. The bullet was barely stuck and easily pushed out with a cleaning rod and wouldn't of allowed me to chamber a second round, on the S&W the 9mm win bullet went about 2" down the barrel before it stopped. Had I not checked to see what had happened I would of blown up that pistol.