https://www.vintageguns.co.uk/magazine/cartridges-for-two-inch-12-bores

Henry Sharp, Shooting Times, April 1, 1939
Those who are desirous to get the maximum success out of these shorter cases will be pleased to learn that our British ammunition firm, I.C.I., are continually making experiments to ascertain the best combinations of powder charge and shot load for those who demand these shorter cases. For some years I.C.I. have been supplying a 12-bore two inch cartridge loaded into a 5-16 unlined quality case and the load used has been 26 grains of Smokeless Diamond and 7/8 oz. of shot.
Recently, after a considerable amount of experimental work with heavier powder charges, good results have been obtained with a load of 28 grains of Empire powder and 7/8 oz. of shot in a 5/8” deep brass or deep shell quality case. The 5/8” deep shell case has the advantage of allowing slightly more variation in loads as compared with the 5-16 unlined case. Tests with the new deep shell cased cartridge gave even and well-distributed patterns with good average ballistics.
This latest issue with its load of 28 grains of Empire powder and 1 oz. of shot will be obtainable for the coming season as an alternative to the cartridge with the original loading of 26 grains of Smokeless Diamond and 7/8 oz. of shot.

Curtis’s & Harvey “Diamond Smokeless” (introduced in 1903) and Nobel’s Explosive Co.'s “Empire” were both 11 grains/dram equivalent
26 grains = 2.36 Dr. Eq so roughly the standard (British) 20g 7/8 oz. 2 1/3 Dr. Eq. load
28 grains = 2.55 Dr. Eq. so roughly the standard 16g 1 oz. 2 1/2 Dr. Eq load

The standard "light" 12g turn-of-the-century loading was 1 oz. with 40 grains of 14 gr./dram eq. "Schultze" or "E.C." = slightly more than 2 3/4 Dr. Eq.

Still haven't found confirmation of the pre-1954 Service Pressures for the 2" loads
After 1954 the Max. Service Load was 2 3/4 tons = 8,120 psi = 560 BAR
Burrard stated in 1955 the 12g 2” 7/8 oz. 2.36 Dr. Eq. standard service was 6,440 psi and max. service 8,960 psi

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