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.
