Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (C.I.P)
http://www.cip-bobp.org/ Standard CIP proof pressure of 850 kg per sq. cm. (BAR) = 13,920 psi for a service pressure of 10,730 psi
Shotgun recommended minimum wall thickness for Category 1 “Standard Steel” (Non-alloy AISI 1045) with tensile strength = 101,526-123,137 psi
10 & 12g...........................20g
End of chamber - .079”........0.075”
4” from breech - .075”.........0.071”
8” - .043”.........................0.041”
12” - .030”.......................0.028”
16” - .024”.......................0.022”
20” - .022”.......................0.020”
Category 2 (Non-alloy AISI 1060) and 3 (Alloy with 3% Chromium and 2% Molybdenum) with a Tensile Strength of 116,030 - 159,396 psi
10 & 12g End of chamber - .063”
20g - .059"
The Hunter's Encyclopedia, Raymond R. Camp Editor, 1948
From the German proof house: minimal wall thickness at the end of chamber, regardless of length, for 12, 16 & 20 gauge guns should be 2.3mm (.0906”) for “Ordinary Good Steel”; 2.1mm (.0827”) if a “Special Steel”.
Category 1 would fit the industry standards for 4140 & 4340 - both are "low alloy" steel
AISI 4140 Chrome Moly
Carbon - 0.38 - 0.43%
Manganese - 0.75 - 1.0%
Nickel < .01%
Chromium - 0.8 - 1.1%
Molybdenum - 0.15 - 0.25%
Ultimate tensile strength - 95,000 psi
AISI 4340 Chrome Moly Nickel
C - 0.38 - 0.43%
Mn - 0.6 - 0.8%
Nickel - 1.65 - 2.0%
Cr - 0.7 - 0.9%
Mo - 0.2 - 0.3%
Ultimate tensile strength - 108,000 psi