What Bruce said.
A Parker Service and Proof Load table was published in the 1930s and reproduced in the
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.
Of course by the 30s the remaining "time bomb" Damascus rough forged tubes had been destroyed
Parker- America's Finest Shotgun by Peter H. Johnson
All of these barrels and tubes used by the Parker were made in and around Liege, Belgium. So far as the knowledge of still-living Parker officials goes, none were ever successfully made in the United States. These barrels, which were imported as "rough tubes," with a low tariff were reasonable in price and very beautiful; but with the advent of progressive burning powder after World War I they were doomed as they were unsafe to use with these new and more powerful powders. So at that time Parker Brothers, certainly not to be left behind by such a plain and inescapable necessity, discontinued the use of them and started making their own barrels from the latest American steel that had been developed during World War I. It is interesting but hardly surprising to note that when Parker changed from figured barrels to those made of fluid steel the factory officials destroyed under a hammer all the barrels that they still had in inventory rather than run the risk of these barrels ever being used.