There are failures caused by overloads, but most are tracked down to bore obstructions.Even failures in or near the chamber can result from obstructions, such as 20 ga shell in 12 ga or loose base wad from previous shell.
Mike
In the case of the blown Fox I saw, I would have initially voted for bore obstruction, probably a base wad, had the shooter not checked his reloads later and found a couple with no powder charge. That makes a double charge more likely, although it still could have been a base wad.
Re reloading, I'm aware of at least one modern double gun (from a "name" European maker) that blew from a factory shell. Inspection revealed that incorrect barrel boring was the problem, with a very thin spot (I don't recall for sure, but I seem to remember something closer to .010 than to .020) where it gave way.
