re: bore measurements
After the proof house revisions of 1887, the British Proof House began using fractional bore
So a British 12 bore (and marked '12') could be .729" - .739". If now .740" or > .010" larger than at proof, the barrel is considered out of proof.
The relevance to our U.S. maker's vintage doubles is that a bore greater than expected, esp. with a "mirror" bore, has very likely been honed; making accurate measurements of wall thickness from breech to muzzle imperative.
1954 - 1989 the bore in inches was marked and chamber length in mm, but still with the bore in a diamond.
The bore in inches was dropped in 1989 and replaced with mm.
We also know the factory chamber lengths (which varied over time) of our vintage doubles, and if longer than expected then measurement of the end-of-chamber wall thickness is critical.
Threaded Mode