I see what you are saying, CW.
I think that the back thrust on the breech-face is the problem. On the original Contender, it would handle high pressure .223 but not 22-250 due to the larger “piston”(case head) impinging on the breech-face.
The original Topper rifles (pre- SB1or SB2) were limited to 30-30, .22 Hornet, .44 magnum etc.
So, I am assuming correctly or otherwise that I shouldn’t build a .360BH from a 20/12gauge Topper.
Hmmm,... Topper. I read that but wasn't absorbing it. I'd spare the Topper that stress too.
There's a big difference between the 42kpsi 30/30 and 50 or 52kpsi for the BH. No sweat in an SB2, but it's not an SB2...
It was what, ten years ago those started going through the roof? I remember guys selling SB2 receivers really cheap and almost giving SB1s away.