Read the item description!
Let the ideas roll. One thing that is obvious. He doesn't know what happened, gives a list of possibilities that would all have been user error and then bashes the workmanship of the gun.
I feel bad for the fella but.........if you were to slam your wanky in the car door it wouldn't be right to blame the folk in Detroit for workmanship....and no I would not buy the car door so that you could get a new wanky.
Of course the barrel threads are flat! They were just stripped out of the receiver.
The fired load "has been confirmed to be 48 grains of IMR 3031 over a 300 grain Barnes bullet". That load is 10 grains below the suggested starting load for that round. It is an overbore case thus most likely it was detonation which is the pilots fault. Always stay between the starting and maximum loads with rifle powders....especially if they are on the slower end of the burning rate for the cartridge in question.
1. The load was reduced thus in the bottom half of the case.
2. The primer flashed through it and didn't get a good burn.
3. The pressure was still enough to push the bullet into the rifling a short amount.
4. Then the rest of the powder lights off(detonates) and spikes at such a rate that the stuck bullet can't get out of the way and the gun goes to pieces.
I saw the same thing happen to a model 70 in 458 Winchester several years ago. The bolt held but the locking lugs were welded into the recesses and the bottom of the barrel split as did the stock. The shooter only had minor injuries but learned not to shoot squib loads with powders designed to operate at higher pressure.
My guess is that 48 grains of IMR 3031 in the 45-70 case was only developing around 10,000 CUP....not at all what that powder was designed for and this created an unstable condition. 10,000 CUP most likely won't blow a gun apart to this extent but the pressure spike from unstable detonation will!
My 2 cents worth!