doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Measuring chambers New idea ???

I picked this up on Ebay some years ago. Didn't pay much as I recall.

It will easily measure to a depth of 6 inches.

Pete

A Graduated Chamber Gauge is Absolutely & Totally Dependant on an exact match of diameters with chamber end. Their use was promoted & advertised by sellers of Chamber Reamers, or Gunsmiths doing chamber jobs. Many, Many a chamber has been reamed & tampered with because it just happened to be a thou or two under-size which was totally immaterial, but they brought a few entrepreneurs a bundle of Cash. Having checked a few chambers short which were not I discontinued use of a "Chamber Gauge" & depend on my Starrett 6" scale, measuring to the shadow line. A $3.00 General scale from your home improvement store will do the same job just as well, I just happen to still have my Starrett. "IF" the gauge is not an exact match to the chamber diameter the scale is the more accurate.
Those internal dial calipers as pictured are very good, but expensive unless one has a lot of use for them. I don't need one that often. Such items were company furnished at work so I didn't have one in my personally owned toolbox.

PS; Many of those chambers I mentioned were "Fixed" because of a chamber gauge, "BUT" they "Weren't Broke".

My plug gauge is made to CIP measurments .If you are talking con men ,any gauge can be used to pull the wool, the more complex the better to fool.

My main interest is L.C. Smiths. In Brophy's book "Plans and Specifications" there are factory drawings of chamber lengths and the taper used. Gives the starting bore size and the ending bore size. For a 12 ga. the chamber length from the begining was 2 3/4" or in some 3" if stamped. Start was .811 and 2.750 later waas .797. Forcing cone taper on all gauges was .014 in 2 3/4".
I'm sure other makers had similar drawings.

I've made these plugs in aluminim and they fit right to the 2 3/4" mark in most guns. The guns they don't fit in were probably polished.

For any plug gauge to work to tell us chamber length, both the chamber and the gauge have to be machined to standards. It is pretty easy to machine the plug gauge to standard with no tolerances, but much harder to ream the chamber to standard each and every time. Reamers wear and have to be sharpened periodically, and the number of times they are/were sharpened might depend on the financial condition of the company, but in any case they would be used sometimes after they no longer met the standards of the time. The taper of the chamber is 0.005" per inch of length. If the reamer has been sharpened until it is 0.002" undersize, a properly sized chamber gauge will indicate that the chamber is o.4" shorter than it actually is. I have two Tobin doubles that illustrate this. Both show a 2.5" chamber when checked with plug gauge, but when checked by another means show that the forcing cones actually begin at 2.75". I have used several means to check chamber length, and usually use the plug gauges first because they are the handiest, but if I suspect some anomaly, I will use another method to verify the gauge. The way I usually check is with a pair of spring loaded round leg inside calipers. When set with a light friction fit in the rear of the chamber and slid to the front, a slight increase in friction is felt until they come to the forcing cone, at which time an abrupt change will be felt. I mark the legs of the caliper and measure with a rule after I withdraw them. I could mark the legs of the calipers with a light nick from a triangular file, and would need no other tool for measuring chambers of any gauge, and all for about $10.

You can pay hundreds for a Starrett or Mitutoyo, but Enco has Chinese knockoffs for a whole lot less money. Accurate enough to take to gun shows or stores to measure guns you are considering. With a 3-1/4" reach, also useful for measuring choke constrictions and tapers. Enco also has a house brand (also Chinese) that is much better quality than their low-end unit, and this one is good enough for the work bench, but I don't remember the brand name.

Would a chamber cast using Cero-Safe work? Cheaper than $100.

I'm talking about gages made to specs. Problem here is so many of the guns we deal with here were made prior to those specs. With a body taper of .005" per inch a mere .001" too small chamber will stop a gage short by .200" or a 2 3/4" chamber will show up as approximately 2 9/16. That .797 JDW mentioned in the L C Smith specs is neither SAAMI nor CIP specs, but simply SS (Smith Specs). I made a very nice set of gages 12, 16 & 20 to SAAMI spec for minimum chamber dia at forward end. I very quickly found that dealing with pre WWI guns I could get a much more accurate measurement of actual chamber length with my 6" scale than with these precision turned gages. This is a case where one size just won't fit all.
"IF" one wants to get arrogant as to the ""ONLY"" way to do it then what ois really needed is some cerro-safe casting alloy & a shadow-graph to read the cast. With this set up you can find absolutely anythibg about the chamber, bore, cone etc you would ever want to know.
My friend I spent 35 years in a machine shop measuring everything imaginable & I can assure you a set of those gages is not the "Only" way to accurarely measure chamber depths, in fact they are not even the "Best" way, "Unless" you individually build a gage to fit for each & every chamber needed to be measured. But by the time you figure that out you have no further need for the gage.
But Hey, if you want to rechamber your guns from 2 3/4" to 2 3/4" just because one happens to be .001-.002" under the gage size that's fine with me. As to my guns "IF THEY AIN'T Broke I DON'T FIXEM" so I'll just keep right on using my little $3.00 scale.

The British Proof houses use plug gauges. That's good enough for me. As to those who wish to use other methods, then that's down to them .
As to those who question the accuracy due to variation in cutter size or due to wear ,that's why there are tolerances with maximum and minimum's both in diameter and depth.
Shot gun cartridges are not accuratly sized like metalic ammunition so the tolerances are much greater.From memory a 12bore chamber has .010"in diameter and about .100" in depth.

gunman, I agree to what you said. If the reamer did have wear, then the when it came to the inspection team to look at them, it would get sent back.
Most L.C. Smith drawings are three places past the decimal and back when I worked in fairly close tolorence work, three places fast the decimal meant I believe +/- .005, two past decimal was +/- .025. Most of our drawings were stated how much in plus or minus, and some were plus .002, minus .000. Also I believe four places was +/- .0005.
All of our machines were in metric, and usually were one to one on the lathes, which made it easier than in thousandths, but decimals in thousandths was in parenthases