I've seen one, and I've seen the results (to the shooter) of another one.
The one I saw was a Fox 20ga. Much like the first Parker Sherman Bell blew up with his high pressure destruction tests, reported in Double Gun Journal, but in reverse. That Parker blew from the hole where the extractor fits under the rib towards the outside. The Fox in question blew from the hole where the guide pin for the extractor rides, on the outside of the right chamber, peeling back in towards the rib. The only possible obstruction might have been a base wad, but the shooter believes--on checking other shells in the box of reloads he was shooting--that he managed to double charge a shell.
The other one I know of was a modern Spanish double by a well-respected maker. I saw the damage to the shooter's hand. Inspection of the barrels at the point where the failure occurred showed a wall thickness of something like .005. Must've been a barrel boring error that somehow still managed to pass proof.
Stan, wondering about your objection to Unique. I don't use it in target reloads, but I have worked up a 12ga low pressure hunting load (7300 psi, 1 1/8 oz) that I occasionally use for pheasants in a vintage double.
