Thanks to everyone for the contribution, esp. Lagopus.
20,000 psi is interesting, and would have been measured using crushers so modern piezo transducer numbers would be 10-14% higher
John Brindle, author of Shotgun Shooting: Techniques & Technology published a review of Proof and Service pressures in Part 5 of his series in The Double Gun Journal, “Black Powder & Smokeless, Damascus & Steel”; Volume 5, Issue 3, 1994, “Some Modern Fallacies Part 5”, p. 11.
His estimated post-1954 but pre-CIP standard pressures by LUP converted to piezo transducer PSI was proof pressure of 12,250 psi for 2 1/2" and 14,050 psi for 2 3/4"
CIP High Performance (Magnum) Mean Proof Pressure is 1320 BAR = 19,145 PSI, which is about the SAAMI U.S. proof pressure.
