Pasquals Law would dictate that pressure would be equal on all internal parts of the case, but of course, the base is thicker. I think Miller is probably correct about the back thrust, as in hydraulics, force = pressure x area. If the same rules apply to cartridges then a .45-70 at 20K psi would probably stress an action more than a .222 Rem. at 45K psi. I assume double rifles that handle the large Nitro Express cases or .458 magnum class cartridges with pressures exceeding 35-40 K psi must be stronger and have more locking surface than the average double shotgun. I do recall reading that sideclips actually help with breeching strength. I wish I knew more about it, but I guess if it was easy a lot of us with pitted bore L. C. Smiths' and blown up Foxes would be converting them to .375 H&H flanged double rifles. I saw a fairly nice looking .45-70 double rifle built on a Savage 311 on Gunbroker a couple weeks ago. I imagine it might be OK with low pressure factory loads safe for use in Trapdoor Springfields, but not the high horsepower loads I use in a Siamese Mauser conversion.
