Have been out of the loop, but just finished reading this thread in its entirety. W/o getting into the semantics of terminology and zero pretention about anything on my part, I can add this:
Federal Top Gun hulls until very recently used a wound paper base wad, not a card wad, per se. It was from all appearances the same wound paper base wad used in their paper hulled target loads. There are now Federal Top Gun loads using a different hull w/a separate plastic base wad and a larger dia. primer than the original Top Guns. It was not stated which hulls were in use at the time of failure.
On another note, I know of a catastrophic failure that occurred some years ago involving a single bbl. 12ga. gun and a new factory 32gram target load cartridge that was presumably loaded w/a ball propellant. Said cartridge had been on the dash of a farm/ranch truck for several years.. in the triple digit heat of summer and hard freezing cold of winter and rolling back & forth on the dashboard countless times before finally being loaded to dispatch a snake the owner took exception with. After much thot & discussion, some of it here, we likely correctly concluded that the powder had become 'dust' and it's surface area expanded by some large and unknown amount or multiple and the deterrents, if any had been worn off of the powder's granules and the cartridge had essentially become a bomb, exceeding the gun's receiver and barrel strength [design limits]by some margin. FWIW, the owner was not litigious and honestly, it wasn't anyone's 'FAULT'. Rather it led to a better understanding on our part how it possibly came to occur.
I have come to a better understanding, or perhaps I should say an expanded understanding of the term 'seam' in reading this thread, thanks to all the players/contributors here.
Thankful that the owner did not sustain more serious physical damages. As to exact cause(s), it will likely never be more than conjecture. A ring bulge, to me, says obstruction and once catastrophic failure occurs it would not be unlikely for any trace extreme pressures to work on the next weaker link which might be something like a braze line for a side rib in the immediate proximity. I dunno, but if the joint where the barrels were fitted to the monoblock is damaged as stated, essentially pulling them partially from the mono-block(!), you can bet the pressures involved were off the map for design criteria.
