At this point, I'll throw out a wild-assed guess and say he left the gun where mud dauber wasps were able to build a nest in the barrels, or there was some other bore obstruction. The closest I ever came to a Shotgun blow-up was a strange feeling that I should check the barrel on a shotgun I was pointing at a groundhog near my garden. I lowered the gun and eased back into the garage and saw the barrel was plugged solid with a mud dauber nest. I now keep tape over the muzzle of my garage gun.
Second guess would be a problem with the cartridge. The gun was tried and true for 141 years. Every shell you chamber is a brand new unproven potential problem. Even factory loads with strict quality control sometimes get recalled due to screw-ups. Might be worth sending out some unfired shells from that box for pressure testing. At this point, guessing is just discussion.
