Chukarguy: do you not mean 9" from the muzzle for .025" - .030"??

Courtesy of Hugh Lomas for English 12 bore Game guns.
Pre 1925 Proofs:
Chamber immediately prior to forcing cone - .105
9 from breech - .045
4-8 back from muzzle - .025
Prior to 1924, the Belgian and British Service (using) maximum load for 12g 2 1/2 and 2 5/8 shells was 1 1/4 oz. / 3 1/4 Dram Eq. (1220 fps).
The pressure of that load would have been about 8500 psi with BULK smokeless powder; with DENSE smokeless 9,500 - 10,500 psi.

Post 1925 proofs:
Chamber immediately prior to forcing cone - .100
9 from breech - .040
4-8 back from muzzle - .025-.020 minimum
In the 1925 British Proof House revisions, the 2 1/2 & 2 5/8 12g service load was reduced to 3 Drams with 1 1/8 oz. shot with a mean service pressure of 3 1/4 tons = (converted using Burrard's forumula) 9,682 psi.


Commission Internationale Permanente pour l'Epreuve des Armes Feu Portatives (C.I.P) http://www.cip-bobp.org/
Standard CIP proof pressure of 850 kg per sq. cm. (BAR) = 13,920 psi proof pressure = 10,730 psi service pressure (SAAMI)
Shotgun recommended minimum wall thickness (p.4) for Standard Steel with tensile strength 700 - 849 N/sq. mm = 101,526-123,137 psi
http://www.cip-bobp.org/sites/default/files/new_file/A-4-1_EN.pdf
10 & 12g
End of chamber - .079
4 from breech - .075
8 -.043
12 -.030


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