Prof242 is building a 12 bore slug gun for pig shooting, not a nitro or nitro for black Express rifle.

Any 12 bore double of ordinary strength will suffice.

We know that a 3 1/4" case full (140gr.) of 2F BP in .50 shooing a 500gr. bullet will produce 'around' 30,000PSI - A.Arms did the testing, and a .50 2 1/2" case with a 450gr. and 90gr. 2f will produce 28,000PSI - again, A.Arms did the testing, while the short 2 1/10" .45 case shooting 70gr.2F and a 500gr. bullet will produce 22,000PSI. The .45 x 3 1/4", with 120gr. 2f and a 500gr. produced some 32,000PSI for them.

I am not a pressure expert either, but can extropolate trends.

I know from experience that a 1900 era fairly light double 12 can handle a 500gr. round ball and 7 drams of 2f BP and that the pressure generated is slightly less than a modern Trap load (11,500PSI), which is the same as a 2 3/4", 3" or 3 1/2" magnum fired in suitable chambers. It is not the pressure alone that 'springs' actions.

The Baikal or BRNO, or even cheaper Spanish or Steoger double 12's will meet the criteria for the initial post.