1925-1954 the 20g 2 1/2" would be marked "7/8 oz." for a service load of 2 1/2 Dr. Eq. = 1210 fps at 3 Tons (but not marked as such). 20g was frequently loaded with 2 1/4 Dr. Eq. as in the U.S. however.
After 1954, 2 1/2" 20g (still 7/8 oz.) were usually 3 Tons;
2 3/4" 20g were 3 1/4 Tons.
The 1954 Rules of Proof “Highest Mean Service Pressure” equivalent transducer values as converted from LUP by Burrard’s formula:
3 tons = 8,960 psi
3 1/4 tons = 9,800 psi
The 1969
Commission Internationale Permanente pour l'Epreuve des Armes ŕ Feu Portatives standards were not ratified by Great Britain until 1980
http://www.cip-bobp.org/homologation/en/tdcc_public?page=1&cartridge_type_id=7Both 65 and 70 mm 20g standard is SERVICE 830 BAR or 12,038 psi;
MSIP 950 BAR or 13,779 psi;
PROOF 1040 BAR or 15,084 psi.
