Good stuff from 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
......................Standard Service... Max. Service.....Proof
12g 2 1/2”.........6,800 psi.............8,800 psi.........12,250 psi
12g 2 3/4”.........7,800 psi.............9,800 psi.........14,050 psi
(CIP 2 3/4”).......10,733 psi............12,328 psi........13,489 psi
