shrapnel: IMHO there are many on this Forum with the bona fides to provide accurate and reliable information
Actual strength testing of pattern welded barrels
https://docs.google.com/document/d/1cvqRzkg0wEjhAAcFWr8gFi7aPFRsSIJ_hahfDxmrNAU/edit
and vintage steel barrels
https://docs.google.com/document/d/1dnRLZgcuHfx7uFOHvHCUGnGFiLiset-DTTEK8OtPYVA/edit

eeb:
John Brindle, author of Shotgun Shooting: Techniques & Technology published a review of Proof and Service pressures in Part 5 of his series in The Double Gun Journal, “Black Powder & Smokeless, Damascus & Steel”; Volume 5, Issue 3, 1994, “Some Modern Fallacies Part 5”, p. 11.
His estimated post-1954 but pre-CIP standard pressures by LUP converted to piezo transducer PSI

..............Standard Service.....Max. Service.....Proof
12g 2 1/2”.....6,800 psi..........8,800 psi..........12,250 psi
12g 2 3/4”.....7,800 psi..........9,800 psi..........14,050 psi

Great Britain adopted the 1969 Commission Internationale Permanente pour l'Epreuve des Armes à 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 930 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.