4seventy,
Thanks for your thoughtful reply. I had thought, and you confirmed, that the difference must lay in the matter of the, for lack of a better term, "piston" area; i.e., the area of the head of the cartridge case. My approach is to apply a fudge factor to my numbers, and use that in my final evaluation. One thing is evident; it is easier to measure the outside diameter than the inside diameter!

What I am trying to do is understand the forces that are generated during firing, and the stresses that the mechanism must withstand (else we will all be "hoist with our own petard"). To that end, I have calculated the forward thrust on a barrel as a result of a projectile upsetting in the rifling, and friction as it travels in its mad rush down the tube. The formula for that is Tf=Cpsi*Ab*Cf, where Tf is Total Forward Thrust; Cpsi is Peak Pressure; Cf is Coefficient of Friction of the bullet against the barrel. The Cf that I use for my calculations is .36. This is much greater than the generally accepted figure for Cf of Cu vs. steel. My Cf tries to account for the engraving and upsetting forces as well as straight friction.

Here is an example of my calculation: .303 bullet, Area = .0706 in^2, Cf = .36, Cpsi = 65000, Tf = 1778 lbs.

Another example: .375 bullet, Area = .1104 in^2, Cf = .36, Cpsi = 65000, Tf = 2584 lbs.

These numbers seem reasonable to me, at least for design purposes. Please comment on my approach and let me know if you think I'm on the right track.