Been following these threads with some interest and have to say this is some of the best forensic detective work I've seen in a long, long time. Y'all are to be congratulated and encouraged for it.
Now for my $0.02. I believe the consensus has coalesced around there having been an obstruction in the barrel. I recall part of the early discussion being that there was snow in the area when the gun was dropped after the blowout. Could the obstruction, which seems to have been the cause, have been snow itself?
Getting a clot of snow in the barrels is something I'm always watching out for while hunting in winter conditions. It has happened to me - thankfully I caught it before firing - and I can say it does happen without even noticing it.
I've seen lots of barrels bulged or even blown because of snow obstruction - quite a common occurence over here - and I don't believe this is the case.
The snow-caused damage invariably happens either before the choke, or mid-barrel where the barrel walls are the thinnest.
As a matter of fact, not all snow obstruction does damage to barrels (that's why it's so common - people get to be careless). The light, christmass-card-flakes sort of snow hardly ever does any harm. It takes heavy, wet snow to get the job done.
In order to blow the barrels in the chamber area, it would have to take a whole lot of wet snow somehow working its way to the chamber area and freezing in place there.
