Jeff Kuss graciously shared an explanation from Parker Bros. of the testing protocol. It's here about 1/3 down
https://docs.google.com/document/d/1...UOZEFU/preview
It is important to understand that the "rough forged tubes", both damascus and fluid steel, sourced in Belgian by all the U.S. makers were also proved.
A Parker Service and Proof Load table was published in the 1930s and reproduced in
The Parker Story p. 515. 12g 2 3/4” shell Service Pressure is 10,500 psi. Definitive proof used 7.53 Drams Black Powder and 2 oz. shot with a pressure of 15,900 psi. The pressure was no doubt measured using LUP and modern transducer values would be 10-14% higher, or more than
17,500 psi.
LTC Calvin Goddard reported the same numbers in “Army Ordnance”, 1934. He wrote that Parker followed the SAAMI standards of that period: 13,700 psi proof, 9500 psi service for 2 5/8” chamber; 15,900 psi proof, 10,500 psi service for 2 3/4” chamber (by LUP)
+ 10-14% for modern transducer measurement.