British proof works on the internal diameter of the bores at 9 inches from the breach.
A gun goes out of proof if that measurement is increased into the next bore size.
The system does not measure the subsequent loss of wall thicknesses that may result from external striking down, unlike some European proof houses that mark the weight of the barrels at proof and only allow a percentage weight loss before the gun ceases to be in proof.
The good news is that minimum wall thicknesses are generally in the front end of the barrels where the pressure curve is low.
The bad news is that there is little margin for dent raising or bulge striking down in the thin areas.
The Gun Trade prefers to avoid anything under 20 thou for that reason but some lightweight guns (particularly 2” 12 bores) May have been less than that when new.
What this guy said….
.015 wall is pretty thin. It really matters what the measurements of both tubes are at the 9” point, and what ammunition you are thinking of feeding it.
Stan, ever hunt ruffed grouse? Or, how ‘bout pheasants in the cattails, behind a snow storm, when the daytime temps are below zero and you have to kick them loose in front of a Setter or a Pointer?
Cylinder has its place on a 12. Sometimes.
Best,
Ted
