Did British gunmakers adhere to .729" for 12 ga. or was there a range?  Is there anything in these proof marks to suggest exact diameter?  Does the diamond with 12 and "c" in it have significance other than gauge or range of manufacture from 1925-1954?  Thanks.  Gil

12 C inside the diamond along with the 2" tells you it has a standard 2" 12 bore chamber.

The bore diameter is marked as 12, meaning at 9" forward of the chamber it would accept the 12 bore plug gauge but not the 11 bore plug.

I have an older 12 gauge Birmingham shotgun 1904-1925 proofs marked 13/1, so there must be a range? What does the 13/1 signify?

13/1, I am going from memory, means the original bore diameter would take the 13/1 plug(.719) but not the 12 plug(.729).
Steve

Gauge was marked starting with the 1855 Rules of Proof

Bore dimensions and marks post-1887 Rules of Proof
"At proof if a plug gauge of 0.729" diameter (but not one of 0.740") will enter the bore to a depth of 9", that barrel would have been marked 12."



Bore was marked in metric starting in 1984 to the 1889 Revisions.

Thanks, Steve, so is another way of stating what you said is 11" from breech on a 2" chamber a 12 ga. plug would fit, but not a 11 ga.? Or is it 9" forward of the breech? Gil

No. The measurement is taken 9" from the breech. Chamber length is not a factor.

A bore marked 12 measures between .729" - .739"
A bore marked 13/1 measures between .719" - .728"
A bore marked 13 measures between .710" - .718"
Gunmakers would generally bore the barrels in the low to middle of the range so there is room for honing without the need for a re-proof. This is not always the case though.

Blue Grouse is correct, my mistake.

I have seen doubles with one barrel stamped 12 and the other stamped 13/1. One of these was a Boss, so the difference must not have been considered a lower standard.

Thanks. Gil