doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com "Measuring" chamber length

I was recently looking at an English gun. It was marked on the barrel flats "2 1/2" and 1 1/8 oz." Ever the skeptic, I had the owner measure the chambers with his Galazan flat-leaf chamber gauge. They measured 2 3/4". I used my own gauge, a cylinder-type made in England and it showed 2 1/2"!

Yikes, is this gun in or out of proof?

With all the discussions on this board about guns in and out of proof, shooting 2 3/4" shells in 2 1/2" guns, impacts of potential value, etc., how is one to tell exactly what length the cambers REALLY are on a gun?

Someone in an earlier post said that he made a chamber gauge with an end diameter of .774". Mine measures .800", who knows what the Galazan gauge measures. This may be significant.

If not all gauges are of the same end diameter, how can we possibly know what's what?

I understand that chambers are tapered, wider at the breach end, BUT:

1. What exact diameter should the forward end diameter of a chamber be?
2. Is there a standard measurement in English guns?
3. Is the diameter consistent between European, USA and English guns?
4. Does a gun chambered for 2 1/2" shell have a chamber 2 1/2" long from the breech face to the end of the chamber?

I know that there is a lot of variance in dimension of chokes, therefor I always measure them in thousandths, not relying on the terms IC, M, IM, F, etc. But with such inexact chamber gauges, do we really know what our guns are chambered for, and whether they have been opened or not?

I asked for the chamber front end diameter. Our late friend Oscar gave me .798"(12 ga) that has worked for me. Chamber length is measured from the breech end of the barrels to the beginning of the forcing cone.
I don't really understand your last paragraph. Chokes are indeed measured in thousandths of an inch. The bore I.D. must be known. Subtracting the choke I. D. from the bore I.D. gives you the choke constriction. Verify it on paper patterns if you like but the accepted measurements will satisfy most people.

"I asked for the chamber front end diameter. Our late friend Oscar gave me .798"(12 ga) that has worked for me. Chamber length is measured from the breech end of the barrels to the beginning of the forcing cone." Okay, .798" works for me.

Sorry about the confusion. Yes, I agree that a specific measurement does indicate a degree of choke and most people have accepted that approx. .010 is IC, .020 is Mod., etc.

I was wondering what the commonly accepted chamber diameter is and why there appears to be such variances in the gauges that measure it, leading perhaps to erroneous assumptions of chamber length.

But, perhaps I'm full of it........

There's GO NOGO and then there's mensuration. Wilhelm Gregor's "20 in a 12" hull lashup has always worked pretty well for me. If the rim of the 20 ga hull is just seated on the open crimp of the 12 ga hull and a single wrap of metallic tape added, the diameter is pretty close to .775" and the overall lgth. right on 2.75" [Win AA plastic 12/2 3/4] whereas the AA hull by itself is 2.715" overall lgth open. With the 20 rim pushed into the crimp of the 12, the diameter is .790". Something to fiddle with anyway and you don't need a lathe.

jack

If a 20 ga. rim is .775", it will go somewhat into the forcing cone of a 12 ga., giving a misleadingly long chamber length. 798" will stop at the beginning of the FC, in a normal 12 ga. chamber.

Please note the .774" dia gage I mentioned earlier is "Not" for a chamber length gage. Most current specs seem to call for .798" as the minimum dia for forward end of chamber. A drawing from the 1950's era by noted Gunsmith Roy Dunlap cited chamber specs for 12ga 2 3/4". Minimum chamber called for the .798" dia to occur at min length of 2.614" from breech & with a 5° forcing cone. The .774" was a "Mathamatical Calculattion" of the dia of the "Forcing Cone" in a gun having this minimum chamber at 2.750 from breech. I suggested it might well be useful to have a gage of this dia to & to insure it would go into a bbl to at least the length of the fired length of shells one was contemplating using. This would catch those chambers having very short, steep cones which should likely "Not" be fired with shells longer than actual chamber depth. This gage would thus be used in "Conjuction With" a chamber gage, not as a chamber gage itself. I generally check chamber depth (even though I turned a set of 12, 16 & 20 on the lathe) with a 6" machinist scale. From my limited experience in this it seems the "Tight" chambers are more prevelent on older American & Continental guns than on English ones. This is the reason though I usually just use the scale.
I tried the 20 in a 12 & to me it seemed useless as the 20ga rims I checked were .760" or less.

I dont have a copy of the 1954 Rules of Proof in front of me but I can recall procedure and process.
The forward end of a 12 bore chamber should be .800" dia min (max .810) when finished (*see note below)This is constant regardless of length.The chamber should taper at a rate of .005" per inch of length.
A plug of the minimum dia should enter to the nominal length i.e 2.5002 for chambers marked as 2&1/2" or 1&1/8th oz.It should not enter to a depth of more than 2.550"
NOTE: A Gunmaker may submit a gun up to .003" undersize to allow for final polishing. I interpret this as only applying to new guns or rechambering jobs with the onus clearly on the submittor to polish this out after proof.
General observation: Proof inspection of this has been sporadic to say the least.I have had 80 year old Greener guns that would only accept .800 plug to 1&3/4".Chambering reamers were relatively difficult to make and I suspect were resharpened causing a loss of dia over time,also it should be noted that standardization only took place immediately prior to WW1 in UK.
Hope this helps

Courtesy of dt Perhaps someone could supply the various dimensions

I won't/can't comment on the accuracy of the tools used but I use a Professional set made by Galazan that has a collet for each gauge that threads on an insertion rod with markings for chamber length. The set is hardened and works quite well.-Dick

Mark,

I have a set of the "Brownell's Chamber Gauges" from 10ga down to 410ga, and I just miced the 12ga one and it measured, .799 at the forcing cone end, and length of 2.630 for 2 1/2" shells, 2.755 for 2 3/4", and 3.505 for 3".

If anyone would like the measurements on another gauge, email me and I'll measure it for you, and email you back.

binko