Since it was a powerbelt... and this is just a thought as they never move off the charge on me..

Perhaps he reloaed with the powerbelt, walked around all day, put the rifle in the trunk, all the bouncing around moved the powerbelt off the load, he never checked the load before he fired to clear it, and since the powerbelts was off the load, he had a big air space and a technically, barrel obstruction. That would account for why the rifle blew near the breech where his hand was, and why the load failed to move the projectile properly up the barrel..

I'm not a firearms expert, but if someone asked me what I thought happened, that would be my reply.