I had a chance to look at a c. 1910 Cashmore sidelock damascus 16 ga. gun whose subsequent nitro-proof marks are Birmingham House' post-1954'. The right barrel is stamped .662" and the left at .669" in accordance with that 1954 change that required this stamping, among other things. The nominal bores, however, measure .004" beyond those markings nine inches from the breech.

My question is this: Did the Birmingham proof house likely stamp the actual measured nominal diameter of each bore on its respective barrel flat....or are these figures of .662" and .669" indicative of the lower limits of their particular bore range? I ask this because a true 16 bore under the British system is reported to be .662" to .668" in some of my literature....while a .669" begins the 16/1 bore range.

It's very possible that the stamped measurements ARE the actual measured diameters nine inches in, and that the bores have been honed a few thousandths each to .667 and .673 respectively. Regardless, the gun is still in proof with a minimum wall thickness of .029"R and .034"L so it's more a matter of curiosity on my part than anything else.

I'd be appreciative if anyone can shed some light as to whether Birmingham's bore diameter stampings post-1954 are hard, factual measurements, OR rather the lower limit of a bore range that brackets the actual diameter?

Thanks in advance,
Rob Harris

Rob, the information I have also indicates that 16/1 range begins at .669. Can't help with the rest.

Rob;
I know that under the old system of stamping gauge a gun was considered "In Proof" until it took the next larger gage. Thus a gun stamped 16 (.662") was "Out of Proof" if it took the 16/1 (.669") plug. Under this system if a new gun was .668" it would be stamped 16 & taking a mere .002" out would put it out of proof.
Basically it was my understanding the reason for the change was to show actual dia at proof & allow .008" (.2mm) of mat'l to be removed without taking it out of proof.
I am however unable to confirm this to be actual fact.

Thanks fellas for what you contributed here. What you said re: the reason for change makes sense, Miller.....and I've the phone number for the Birmingham House if it gets to bugging me too much. If I make that call, I'll post what I find out. In any event, I appreciated your thoughts.


Rob