That is one of the hazards of plastic framed pistols; blow a case head in a 1911 or other metal framed gun and the gun usually survives; usually not with a plastic frame.
If the charge varied a lot, probably either some of one charge got carried over into a 2nd charge, but also possibly it fired out of battery.
But anyway...Did the OP ever buy one of the AR57 uppers? I've been interested to hear how they shoot.
