Considering an Auto-5 is one heavy receiver, that was a heck of a load!
Now, what if he got some 16 gauge rounds into his package?
I've heard ome 16's will go all the way down to the forcing cone of a 12 gauge with some force- the chances that this happened might be better than the 20ga theory. I'd put money on the shell separation theory myself as stated above.
Another good reason not to use any hull left outside in the rain or exposed to moisture (especially if it has a paper base wad) or attempting to use hulls past their lifespan.
It also doesn't look as though it fired out of battery, does it? I thought about that as a possibility since I have seen a model 12 do this- after it was "home gunsmithed."
But, I don't think this is what happened here.
Look closely at the barrel pictured and tell me, does it look bulged just ahead of where it split? It does to me.
Evidence of an obstruction of some kind.
Poor feller.
