20g service pressures under the 1896 Rules of Proof
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA296&dq
Pressures 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

Frederick Toms, Sporting Guns and Gunpowder, 1897
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA132
Average pressure of Nitro powders
20g 7/8 oz. 3 Dr. Eq. – 3.56 Tons = 10,840 psi
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA158
20g 2 1/2 Dr. Eq. and 1 oz. – 3.07 = 9,195 psi

https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA325
20g with 7/8 oz.
35 gr. Bulk (2 1/2 Dr. Eq.) – 2.39 = 6,910 psi
22 gr. Dense – 2.93 = 8,725 psi

“The Long 20 Bore Cartridge”, The Field in Forest & Stream, October 17, 1908
https://books.google.com/books?id=ejQevDPMUIYC&pg=PA630
2 3/4” case with “Schultze” Bulk Smokeless (42 gr = 3 Dr. Eq.)
The standard 2 1/2” case was loaded with 33 gr. “Schultze” = 2.36 Dr. Eq. (about 2 3/8) with 13/16 oz. shot.
36 gr. = 2.57 Dr. Eq. with 15/16 oz. = 4.65 tons = 14,504 psi (using Burrard’s formula)
37 gr. = 2.64 Dr. Eq. with 7/8 oz. = 4.54 tons = 14,134 psi
35 gr. = 2.5 Dr. Eq. with 13/16 oz. = 3.45 tons = 10,472 psi

William Bruette, Guncraft: Guns, Ammunition, Wing & Trap Shooting, 1912
https://books.google.com/books?id=5g51K93as84C&pg=PA210&dq
20g 2 1/2” case 33 gr. Curtis & Harvey's Amberite Bulk Smokeless (2 1/3 Dram) 13/16 oz. = 11,648 psi

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