No CIP signatory has a MWT
requirement for proof
Commission Internationale Permanente pour l'Epreuve des Armes ΰ 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, which Chrome Moly 4140 just reaches
http://www.cip-bobp.org/sites/default/files/new_file/A-4-1_EN.pdf12g...........................................................................
20gEnd of chamber - .079..................................................075
Just past the forcing cone/ 4 from breech - .075................071
8 - .043..................................................................041
12 - .030.................................................................028
16 - .024.................................................................022
20 - .022.................................................................020
British Rules of Proof: If a plug gauge of 0.729" diameter will enter the bore to a depth of 9", that barrel is marked 12. If so marked and a plug gauge of 0.740" will enter the bore to a depth of 9", the barrels are "out-of-proof".