The whole breech pressure thing is going to open up a can of worms here as there are several opinions on this matter here in relation to converted weapons.

It is not the breech pressure that is important up to a degree dependent on barrel wall thickness, but how that acts on the breech face and attempts to open the gun. So BREECH THRUST

FROM THE DATA I HAVE: Base thrust:

400/350 16 Tons
9.3 x 74 16 Tons

30/40 CIP Max ave pressure 47,000 psi
Base area = 0.1643( {.457/2}{.457/2}x 3.14) Inch square x 47,000 = 7722.1 lbs / 2240 = 3.447 Tons thrust.

Now this is not exact there are some issues around this, however even allowing for error of 100% it is for me, a good chioce on a light action for plains game and US use..

I think that the 30-40 is the sleeper cartridge for sane conversions. Wide choice of bullets, plentiful and cheep cases, easy barrel supply and nice to shoot in a lightish gun.

Regards