The British and Europeans have designed a system to confuse we Colonials
and all of this probably won't help
3 1/4 tons is the standard British proof for 12g guns with 2 3/4" chambers. The use of Tons however was discontinued in 1989, and pressure is now reported in BAR. Unfortunately, there is essential NO formula to convert BAR expressed as LUP (CIP/European) to PSI (SAAMI by piezoelectric transducer measurement).
Per the 1954
British Rules of Proof the equivalent CIP service (using) pressure for 3 1/4 tons is 9,682 psi BUT this is also measured by lead crusher units (LUP) rather than transduced psi (SAAMI), which would be about 1000 psi higher (see below).
However, the CIP standard proof pressure of 850 kg per sq. cm. (BAR) = 13,920 psi proof pressure=
10,730 psi service pressure (SAAMI) because the SAAMI psi conversion for the 1954 TONS measurement is different from the 1989 BAR measurement
12g 2 3/4 inch 3 3/4 dram equiv. 1 1/4 ounce loads (1330 fps) are usually near the SAAMI max. of 11,500 psi.