Like all things British, it's complicated to us Colonials
We KNOW some 20g service pressures under the 1896 Rules of Proof
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA296&dqPressures were measured using crushers (LUP) and reported in pounds/ sq. inch; modern piezoelectric transducer pressures would be
10 - 14% higher.20 bore Service charge = 7/8 oz. 2 1/2 Dr. Eq. (1210 fps) = 34 gr. Bulk Nitro powder
C&H “T.S.” No. 4 BP - 8,240 psi; “Schultze” - 8,220 psi; “E.C.” - 9,100 psi
1925 - 1954 barrel flats were marked with chamber length and service load oz. and service load pressures are assumed to be similar but I have no published confirmation.
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 pressures by LUP converted to piezo transducer PSI of the standard
LOADS offered:
......Std. Shell 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
16g 2 1/2”....7,300 psi.....9,300 psi.....13,150 psi
16g 2 3/4”....7,800 psi.....9,800 psi.....14,050 psi
He did not estimate 20g shell pressures I believe the non-deep thinkin' version would be what Larry said; 3 Tons are 3 Tons regardless of the gauge, and 3 Tons = 8,938 psi; or 8,960 psi per Burrard
