It is terribly confusing
This is the earliest pressure numbers expressed as PSI in U.S. literature that I have found; in
The Overland Monthly, Oct. 1895 “Smokeless Powder For Shotguns” p.453
http://books.google.com/books?id=Wv0...J&pg=PA446&lpg
3 1/4 Dram Eq./40 grains DuPont Bulk Smokeless 1 1/8 oz. (1255 fps) = 7440 psi
3 1/4 Dram Eq./44 grains “E.C.” Bulk Smokeless 1 1/8 oz. = 7584 psi
10-14% for modern piezo transducer numbers
This is a Winchester Factory Memorandum dated February 6, 1900
12g 2 5/8" Leader with 1 oz. shot
40 gr. DuPont Bulk = 3 1/4 Dr. Eq. = 2.7 tons/sq. inch = 7952 psi by Burrard; only 6,650 psi if X 2240 + 10% and that is far below other reported numbers.
The Parker DuPont Ballistic Table lists DuPont Bulk 3 1/4 Dr. Eq. 1 oz. at 3.17 tons = 7800 psi by X 2240 + 10% which seems in keeping with other numbers.
42 gr. Laflin & Rand Bulk = 3 1/2 Dr. Eq. = 3.3 tons = 9968 psi by Burrard; 8,130 psi by X 2240 + 10%.
12g 2 5/8" Leader with 1 1/8 oz. shot
40 gr. DuPont Bulk = 3 1/4 Dr. Eq. = 3.9 tons X 2240 + 10% = 9,600; 11,984 psi by Burrard. The Parker table lists 4.0 tons X 2240 + 10% = 9,850 psi
Either the British and U.S. ballistic engineers were using different Tarage tables, or DuPont was using a PSI Tarage table and simply converting to Tons by multiplying X 2240??