1954 Rules of Proof :

Max length of chamber 2.100" Min length of chamber 2.000"

Highest Mean Service Pressure 2 3/4 tons at 1" and 1 3/4 tons at 6" (measured by radial method)

For The Proof of Barrels of the First Class :

12G 2" Provisional Proof 266 grains T.P. Powder with 1 1/4 oz No 6 shot (lead obviously)

Definiutive Proof 170 grains T.S.P. Powder with 1 5/8 oz No 6 shot

Even allowing for the vagaries in "strength" of BP as used in Birmingham Proof House and that available today, I think that the figures above provide as much information as necessary for judicial home loading.