Happened to my first shotgun, blew the rib off, opened the right barrel 1/2 inch at the breech end and tapered like a carrot towards the muzzle. Never unlocked the barrels, when I opened it up all that was left of the shell was the brass head. No one was hurt but it sure was LOUD with my right ear so close to it.
p.s. I still have the barrels to remind me