It would have been helpful, prior to the standardization of US gunmakers on 2 3/4"/3" chambered guns, all of which--within a given gauge, except .410--have the same service pressure, if the industry had used proofmarks to indicate the standards under which their guns were proofed. There were no industry-wide standards prior to SAAMI (mid-20's), and even after that, we had a bunch of different chamber lengths for different gauges. And, from what I can find in the literature, different proof and service pressures. All of which wasn't much of a problem as long as appropriate factory ammo was readily available--which it was, at least up to WWII. After that, shorter American factory shells disappeared pretty quickly, and a lot of gunsmiths dealt with the problem by lengthening 2 1/2, 2 9/16, and 2 5/8" chambers to 2 3/4", and acting as if that solved the problem. Which it didn't then, and doesn't today on old American guns which have been rechambered--any more than taking a 2 1/2" Brit gun and punching it out to 2 3/4" means that you should use any and all American 2 3/4" factory loads in that gun, even if it's cleared 850 bar/"standard" CIP proof. The advantage of the British system is that we at least know when the gun has had its chambers lengthened, and that it has been reproofed. But that only happens because it's all mandatory over there.
