The logic that I have used as a guide for action strength was to multiply the cross sectional area of the shell it was built for times the working pressure of that type of shell.
Do the same for what you intend to shoot and see how the numbers compare.
For example a 12 guage at 12000 psi (guessing a base diameter of .8) works out to just under 7700 I think. A 45-70 base of .502 and black powder pressures in the 25 - 30,000 range gives about 7500 units.
In other words as long as you stay below roughly 30,000 psi loads, the thrust on the breach face should be roughly similar to modern shotgun loads. On the other hand load the same shell up to Ruger #1 pressures and you will almost double the thrust, at least in my opinion.
cheers Doug
