Sporting Guns and Gunpowders: Comprising a Selection from Reports of Experiments, and Other Articles Published in the "Field" Newspaper, Relative to Firearms and Explosives, Volumes 1-2, 1897
https://books.google.com/books?id=inQCAAAAYAAJ

“New Rules of Proof”
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA7&dq
Under the 1896 Supplementary Nitro Proof Rules, the Nitro Proof Charge was not fixed. It could be changed by the Proof Houses as existing powders changed or new products released. The Proof Houses would determine the charge in order to keep the pressure 80 - 100% above that of the Service Charge.

Curtis and Harvey’s No. 2 (“T.S.2”) was a fine grain fast burning Black Powder similar to GOEX FFFg that produced pressures substantially above that of the standard proof Black powder.

Nitro Powder Comparison
S.S. was a Bulk Smokeless nitro powder made by Smokeless Powder Co.
https://books.google.com/books?id=AjsoAAAAYAAJ&pg=PA468&lpg
It was discarded as loading with higher charges of powder produced significantly greater pressures than “E.C.” or “Schultze”.



Black and Nitro powder Service and Proof Charge Pressures
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA296&dq
Experiments to ascertain the pressures given by service charge and proof charges used in England have been carried out by the Field. Under the existing Regulations of Proof in England, a “service charge” is specified for every bore, and the compulsory definitive proof is made with double the service charge of proof-house powder and 1 1/3 (times) the service charge of No. 6 shot. Besides the compulsory proof, a supplementary proof with other kinds of powder is permitted, if applied for. Many other charges, both larger and smaller than the officially stated “service charge”, are in constant use. We give records of pressures of a dozen varieties of powder, with the most commonly used charge for 12-bores, as well as with the “service charge” laid down in the Proof-House Regulations.



Nitro 1 1/8 oz. 3 Dr. Eq. (1200 fps) 5330 - 6110 psi
Nitro 1 1/4 oz. 3 1/4 Dr. Eq. (1220 fps) 6360 - 8620 psi
PLUS 10% by piezoelectric transducer measurement


For comparison, The Overland Monthly, Oct. 1895 “Smokeless Powder For Shotguns”
http://books.google.com/books?id=Wv0MAQAAIAAJ&pg=PA446&lpg
3 1/4 Dram Eq./40 grains DuPont Bulk Smokeless 1 1/8 oz. = 7440 psi
3 1/4 Dram Eq./44 grains “E.C.” Bulk Smokeless 1 1/8 oz. = 7584 psi
PLUS 10% by piezoelectric transducer measurement.

“Powder Pressures At Different Parts of the Gunbarrel, 1895”
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA32&dq

Smokeless Powder Pressures

The choice of 32 grains of Ballistite is confusing, as 24 grains is the published load for a 3 Dram equivalent; 26 grains for 3 1/4 Dram.
Alfred Nobel's 'Ballistite' was introduced in 1887, patented in 1888, then ‘Sporting Ballistite’ (for shotgun shells) was patented in 1889, but not released to the trade by Nobel’s Explosive Co. until 1895. The published summary of the Trial states ‘Ballistite’ rather than ‘Sporting Ballistite’.



1 1/8 oz. #6 shot at 1220 fps
Using the estimated Lead Crusher Pressure (Cp) conversion to PSI (pound force per square inch) (Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI
Note: Modern transducer psi measurements average about 10% higher than LUP

32 Gr Ballistite (1 1/8 oz. at 1232 fps = 3 Dram Equiv.)
Breech - 2.9 Tons per Square Inch (UK, Long) = 8624 psi
(2.9 x 1.5 = 4.35, 4.35 - .5 = 3.85 TSI, 3.85 x 2240 = 8624 psi)
2 1/2" - 2.2 Tons = 6272 psi
6” - 1.2 Tons = 3718 psi
12” - .81 Tons = 1602 psi
18” - .33 Tons
24” - .21 Tons

42 Gr Schultze 1 1/8 oz. at 1220
Breech - 1.69 Tons = 4558 psi
2 1/2” - 1.49 Tons = 3886 psi
6” - 1.36 Tons = 3450 psi
12” - .92 Tons = 1971 psi
18” - .55 Tons = 1178 psi
24” - .32 Tons

The 1896 Rules of Proof were fairly short-lived, compared to the "life expectancy" of British rules of proof in general. One interesting part of those rules was that they included a proofmark that gave the precise maximum load data, in terms of both shot and powder charge (both the weight and the type of powder). Although some post-1904 guns had a similar proofmark, most were just marked "nitro proof" along with the service shot charge.

Indeed, and very helpful.
1896 Lancaster marked 42 grains = 3 Drams "E.C." with 1 1/8 oz. shot. And we don't have to guess the pressure of that load - 5330 psi plus about 10%.