Sounds like a bore obstruction to me, likely short-seated bullet or maybe even the old ramrod left in the barrel stunt. Lots of stuff can happen to folks who don't focus on the job at hand.
Sam Fadala did some bore-obstruction demos years ago, mainly to show what would happen with various obstruction scenarios. He used copper pipe, I believe, not gun barrels. Even though BP operates at relatively low pressure, if that pressure doesn't have an escape route, something's gonna give.
