Jack, SAAMI maximum average working pressure for 12 gauge is 11,500 psi (as stated above).

A proof pressure for SAAMI loads of 1.5 x working is between 17,000 - 18,000 psi, hence proof loads of ~ 18,000 psi, as you surmised. This is also essentially equal to the 1,200 kg/sq cm "heavy" proof used optionally in Spain and similar levels available in France and other countries.


Fred