oupa,
I think it is astonishing that the London Proof House would claim to have "proofed" a rifle in such a ridiculous manner.
Seeing that they had no idea what pressure Bramble's cartridge was producing, they also have no idea what pressure their "proofing" was done at.
In reality they proved the rifle at "no idea plus 30%"!
The plus 30% was a guesstimation by the proof house as well,(edited to add) and is incorrect IMO.