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.pdf

12g...........................................................................20g
End 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".

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