Parker 20ga VH with British proofs

[Drew Hause]

"A British shotgun proof rating of 3 tons/sq in (often seen as a "3 TONS" or "3" mark on older shotguns) equates to 6,720 pounds per square inch (PSI). Service pressure= 8939 psi."

David: did you mean a proof pressure of 16,720 psi? Is that by LUP or piezo transducer? Where did you find that number?

Under the 1925 Rules of Proof the Mean Proof Pressure for 12g 2 1/2" & 2 5/8" was 13,700 psi; Max. 15,000 psi
For 12g 2 3/4" Mean was 15,900 psi; Max. 17,500 psi
But those numbers are by LUP and require adding 10 - 14%

The 1954 Rules of Proof “Highest Mean Service Pressure” equivalent transducer values as converted from LUP by Burrard’s formula:
2 3/4 tons = 8,120 psi
3 tons = 8,960 psi
3 1/4 tons = 9,800 psi
3 1/2 tons = 10,640 psi
4 tons = 12,320 psi

Your 8939 psi is the conversion provided by the Birmingham Proof House 11-2001 (courtesy of Larry Brown) and is simply Tons x 2240 x 1.33

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 (TONS was marked 1954 - 1989 and was measured by crushers) but pre-CIP standard pressures 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
He did not record 20g