As far as why it cracked, I think you're on the right track about a bulge being hammered out and the material becoming work hardened.

This one fared better than the one my pal blew up on New Years Eve 40 years ago. Old shells, a dud, he racked in a new one with the usual results. It split down the side. Proof Steel, just not idiot proof.


"The price of good shotgunnery is constant practice" - Fred Kimble