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.
