Whether a gun is in proof is simply a matter of whether the bore diameter has been enlarged beyond its stamped range. So if it is marked 12 it has to be between 0.729" and 0.739".
Also a gun can be taken out of proof by significant alteration of the barrels or action, eg. fitting of interchangeable chokes, conversion to ejector, weld repairs to the action etc.
Pitting, thin walls or poor jointing does not take a gun out of proof how ever bad they may be. Unsafe maybe, but not out of proof.
When a gun is sent up for proof, the proofmaster takes a view as to the safety of the barrels. If he doesn't like the amount or location of the pitting, he may fail it at view and return it for further work.
However, he may allow a gun with significant pitting to go through to the actual proof firing if he feels the gun is basically safe.
I have had guns pass proof with some pitting but I have also had guns fail with only a hint of rivelling.
There is very little apparent logic to the process in borderline cases.
And John is correct, the measurement is 9" from the breech face.
Last edited by Toby Barclay; 05/13/16 02:06 PM.