British 12g 3" 1 1/2 oz. (1925 Rules of Proof "Maximum Service Load") 4 Tons (1954 Rules of Proof Highest Mean Service Pressure) would be 12,320 psi by Burrard's conversion to piezo trandsducers, or 11,917 psi by piezo transducers per the Birmingham Proof House...or something like that
Note the 3" SAAMI Max. is the same as 2 3/4" at 11,500 psi
1925-1954 the 2 3/4 12g max. service load was 3 3/8 Dr. Eq. with 1 1/4 oz. shot with a mean pressure of 3 1/2 tons = 10,640 psi by Burrards conversion.
(Primarily for heavier Waterfowl guns)
Turn-of-the-century U.S. 3" or even 3 1/4" Pigeon loads were limited to 1 1/4 oz. 3 1/2 Dr. Eq. BULK Smokeless at about 11,750 psi /
1 1/4 oz. 3 1/2 Dr. Eq. DENSE Smokeless at about
12,600 psi
