British and CIP load pressures for comparison. See Bro. Vic's comments at the bottom.
The 1954 British Rules of Proof "Highest Mean Service Pressure" equivalent transducer values as converted from LUP by Burrard's formula for 3 1/4 tons = 9,800 psi = 676 BAR.
"Standard Service Pressure" for most loads however was about 8000 psi per John Brindle in The Double Gun Journal, "Black Powder & Smokeless, Damascus & Steel"; Volume 5, Issue 3, 1994, "Some Modern Fallacies Part 5", p. 11.
The equivalent transducer values provided by the Birmingham Proof House is simply Tons x 2240 x 1.33 - NOT simultaneous readings for psi by LUP and psi by piezo transducers:
3 tons/ sq. inch = 8,938 psi
3 1/4 tons = 9,682 psi
3 1/2 tons = 10,427 psi
4 tons = 11,917 psi
Great Britain adopted the 1969 Commission Internationale Permanente pour l'Epreuve des Armes d' Feu Portatives standards March 1, 1980 but continued using Lead Crushers to measure pressure until 1989.
The CIP transducer "Maximal Statistical Individual Pressure" is 850 BAR = 12,328 PSI for a "Maximal Average (Service) Pressure" of 740 BAR = 10,733 PSI, and "Mean Proof Pressure" of 960 BAR = 13,924 PSI.
900 BAR is for a "Maximal Average (Service) Pressure" of 780 BAR = 11,313 PSI and Proof pressure of 1020 BAR = 14,794 PSI.
High Performance (Magnum) MSIP is 1200 BAR = 17,405 PSI for a Service Pressure of 1050 BAR = 15,229 PSI, and Mean Proof Pressure of 1320 BAR = 19,145 PSI.
When a British shell box is labeled:
These cartridges are suitable for use in:
70mm case length: Guns with a chamber length of 2 3/4" or longer, nitro proofed to a service pressure of 3 1/4 tons per square inch (900 kg per square cm)
67 mm case length: Guns with a chamber length of 2 1/2" or longer, nitro proofed to a service pressure of 3 tons per square inch (850 kg per square cm)
Vic Venters, Shooting Sportsman, March-April 2012, "CIP Proof"
"Although the Maximum Mean Pressures for service loads for standard proof (850 BAR) guns are 740 BAR, CIP regulated cartridge manufacturers typically work to lower pressures...between 450 (6,527 psi) and 650 bars (9,427 psi) as measured by CIP piezo transducers."
