doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Chamber length (again)

Last Dollar,
Happy Birthday
Mike

Thanks.It's good to be 75!

What is the correct diameter for a 12ga. chamber gauge? A 16?

I once had a well known barrel man tell me that the reason he measured my 12ga. at 2-3/4" when myself, my gunsmith and the Birmingham Proof House all measured it at 2-1/2", was that his chamber gauge was a different size. He said it all depends on the gauge you use, they are all different.

What horse sh*t you hear in this business...

Chuck with all due respect to Lagopus the MagTech brass shells are slightly shorter than 2-1/2". They shouldn't even touch the forcing cones, much less lodge in them.

Geez, you can lead a horse's ass to water....

Best,

Mike

The length of the chamber is defined by the juncture of where the body cone meets the forcing cone. Quite obviously a chamber gauge, irregardless of its precision in building, can give an exact measurement only if its dia exactly matches that of the chamber's small end. I believe a modern day chamber gauge will be built to the minimum chamber specs according to SAAMI. The problem is these older guns we speak of here preceded SAAMI so don't necessarily meet those specs.
With a dial bore gauge you can observe the slow reduction in chamber size from its taper as you push it along, then see the quick change as the forcing cone is hit. By marking this spot on the handle & measuring from there to the measuring points a very accurate reading can be obtained.
OR, you can do as I do, simply hold the bbls up where their interior is illuminated with the cone seen as a shadow. One can then slide a 6" flexible scale along the chamber wall while observing it. As the leading corner touches that shadow line mark the scale then withdraw & read it. With a surprisingly small amount of practise to insure you are reading consistent you can quite easily read to within the 1/64" (.0156") reading on the scale. Amply accurate for anything you need to know. One of these little scales can be picked up at any home improvement store for a very few bucks, Starrett or Brown & Sharp precision is not really needed here.

My skills are less than many others that have commented on this thread, but I suggest buying a few bucks worth of Cerrosafe and making a Chamber cast. It isn't much trouble and it gives you much more information about the chamber.

It gets complicated, because the Brits and the Americans (for example--and I expect the Europeans as well) used slightly different chamber diameters. Hugh Lomas once showed me a trick for determining whether an original short chamber was lengthened in the States or England. Slightly different diameter of chamber reamer.

That being said, I have yet to encounter a chamber in which one of the standard Galazan gauges caused me to think it was 2 1/2" when it was really 2 3/4", or vice versa. You may come out a hair off, one way or the other, but not nearly that much.

The mechanics of this are quite simple. A normal chamber body is a cone having about .005" taper per inch. The forcing cone will have a steeper taper, dependant upon its length. Consider two gauges, one being .001" larger than the chamber's small end & the other .001" smaller. These two gauges will stop at dtstances rather wide apart. The large one will stop a distance of .001/.005 (.200") short. This would indeed show a 2 3/4" (70mm) chamber having a length of only 2.55" (very close to 2 9/16/ or 65mm). On the other hand the smaller one, assuming a difference in chamber & bore of .070" (about standard if both are nominal size) the gauge would enter into the cone .001/.070 times cone length. For a cone length of ½" the gauge would thus go .007" deep, hardly enough to get up on one's tricycle about.
Not having a Galazan gauge I have no idea to what dinension they finish its diameter. These principals though are plain & simple math & apply whom ever makes the gauge.
Many years ago I personally made a set of chamber gauges to precise dimensions of the minimum chamber spec for the small end given in SAAMI (I wanted to be "Precise"). Very rapidly I found out it was not at all uncommon to insert one in the chamber of an older gun & it show to be short chambered, when in fact it was only a little under in diameter.
There will seldom be an issue of a gauge showing the chamber to be too long, even if it was .005" under chamber dia & the bbl had a 1½" cone length the gauge would only go in .107" too deep. It can be readily seen though if the chamber is smaller than the gauge a large discrepency is seen & a chamber can (& WILL) show to be too short when in fact it is only very slightly small.