Marrakai Thank you for the information, I had only considered breech pressure and not thrust.
I did the math and found these results.
12 gauge .73 rim diameter 11,500 psi max pressure 4810 bolt thrust
50-70 .6630 22,500 2472.57
45-70 .6015 28,000 2532.61
If these numbers are correct a conversion to either 45-70 of 50-70 seems prudent.
is there anything that I am missing, or figured incorrectly?