doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com What's the Bore Diameter of a Gun Marked 13/1?

Is there a table somewhere that lists the ranges for each of the bore diameter markings? I'm looking at a gun marked 13/1 and I don't know what that means it was originally nor does it allow me to determine whether it's still in proof given the current bore diameter measurement.

Thanks.

It would have measured between .719" and .729" at time of Proof. So, it will be out of Proof in British terms if is goes over .729" at 9" from the breech face. Hope that helps. Lagopus.....

Thanks. That's just what I needed to know.

Isn't that .710 to .729?

.710"-.719" is 13/1, not 13... but it depends when it was proofed, because of the introduction of the intermediate bore sizes (when? haven't my refs to hand...); so .710"-.729" is 13 too...! I think...

Edit: getting myself mixed up and will now await correction from a credible source!

I thought:
12/1 is .740
12 is .729
13/1 is .719
13 is .709

I'll assume I'm right until a Brit on the forum quotes the proof house to debunk me.

Here is a handy little reference....

Right when I thought I was getting the hang of this stuff.

As best as I recall the inbetween sizes were started with the proof rules of 1887, so yes prior to that a 13 could have been from .710" to .729" A 13/1 would of course have never been other than .719"-.729"
Gauge diameters are calculated to the nearest .001". The in-between sizes depending on whether there are 1 or 2 steps are either half way or 1/3-2/3 of the difference between gauges. On these any numbers beyond 3 decimal places are simply dropped even if .0005" or above. Thus .710" + .729"/2 = 1.439"/2 = .7195". The .0005 is dropped so the 13/1 is listed in the proof charts as .719". In this case if you calculated 13/1 as a 12� gauge it would figure to .7193" but this is not the case for all gauges & especially with the gauges having two steps.

Thanks. I love how much knowledge is available on this board and the willingness of those who possess it to share it freely.