In the 1925 British Proof House revisions, the 2 1/2 & 2 5/8 12g service load was reduced to 3 Drams with 1 1/8 oz. shot with a mean service pressure of 3 1/4 tons = (converted using Burrard's formula) 9,682 psi.
The 2 3/4 12g max. service load was 3 3/8 Dr. Eq. with 1 1/4 oz. shot with a mean pressure of 3 1/2 tons = 10,640 psi by Burrards conversion.
(Primarily for heavier Waterfowl guns)
It's good to remember here that the "tons" proofmarks didn't appear until the proof rules were changed in 1954. But in spite of the different proofmarks, proof loads really didn't change. This is a quote from Roger Lees, Birmingham Proofmaster both before and after the change from the 1925 to 1954 rules: "The proof loads now set down in writing (in the 54 Rules) are almost in every instance the proof loading in use under the 1925 rules. In general it may be said that under the new Rules of Proof no arm will receive a more severe proof than hitherto." Roger Lees, as quoted by Vic Venters in his book "Gun Craft", p. 206.
