I can't speak to the English proof rules, but SAAMI pressure standards for 2 3/4" and 3" shells are the same in 12 and 20 gauge, and for 2 7/8" and 3 1/2" 10 gauges. Because of this, it would seem to me that the same proof loads would be used for the longer chambered guns as the shorter one. Am I mistaken in this?
Tom, under SAAMI rules, I believe you're basically correct. Since the service pressure is the same for both long and short 10's, the proof pressure should also be the same--although they probably would not use 3 1/2" proof loads in a gun with a 2 7/8" chamber. That would cause a pressure increase in the short-chambered gun.
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