Skeeter, per comments from a previous Birmingham proofmaster, who overlapped the transition from the 1925 rules of proof to the 1954 rules ("tons" era), we know that proof pressures did not change as a result. I've never seen it stated for certain that 2 1/2" service pressure was the same under the 1954 rules as it was under the later 1980's rules (850 bar guns), but like you, I've read on Brit shotshell boxes that 2 1/2"/65MM/67.5MM shells are good to go in 3 ton OR 850 bar guns. That certainly makes it sound as if the service pressure is the same . . . or else those shells are loaded to the old 3 ton specifications, which would mean they're also OK in guns proofed under the 1925 rules.
