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
(CIP 2 3/4”).........10,733 psi...........12,328 psi.....13,489 psi
16g 2 1/2”...........7,300 psi............9,300 psi.......13,150 psi
16g 2 3/4”...........7,800 psi............9,800 psi.......14,050 psi


The case label with a c. 1880 William Sumners, Liverpool 12b specified Curtis & Harvey No. 5 Black Powder:
“Light” - 2 3/4 Dram with 1 oz. shot = 1180 fps
“Medium” - 3 Dram with 1 1/8 oz. shot = 1200 fps
“Heavy” - 3 1/4 Dram with 1 1/4 oz/ shot = 1220 fps
Understanding that pressures generated by Black Powder are lower than that of Dense Smokeless, that would still be a good guide.

Assuming solid stock wood, unmolested barrels and mechanical integrity, I think it would be reasonable to limit a light weight vintage 12g British game gun marked 850 BAR or 3 TONS to loads generating less than 8500 PSI; or 900 BAR or 3 1/4 TONS less than 9,500 PSI or 1 1/8 oz. 3 Dr. Eq. (1200 fps).

These are Ithaca's recommendations from 1901

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And Fox's from 1914

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Standard loads found on L.C. Smith hang tags:
12 gauge.....3 dram.....1 1/4 oz. shot @ 1165 fps (1887 - about 1920)
................3 dram.....1 1/8 oz. shot @ 1200 fps (after about 1920)
16 gauge.....2 1/2 dram.....1 oz. shot @ 1165 fps (introduced 1896)
20 gauge.....2 1/4 dram.....7/8 oz. shot @ 1155 fps (introduced 1907)

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