It was originally proved 1925-1954 with 2 1/2" chambers for 1 1/4 oz.
1 1/4 oz. would be the standard for 2 3/4" chambers for a max. service load of 3 3/8 Dr. Eq. with 1 1/4 oz. shot with a pressure of 3 1/2 tons = 10,640 psi
Bores then were 15/1 or .685" - .692"
It was reproved (Crown over R) 1954-1989 by the London Proof House (the arm and sword over NP, but prior to application of the date code mark which started in 1972) still with 2 1/2" chambers for
THREE Tons; now with a bore of .719". 3 TONS would be the standard for 2 1/2" chambers with a max. service pressure of about 9000 psi
That is substantial honing so wall thickness measurements are critical.
