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thank you,all,very much.
Justin

The bad news for measuring shotgun chamber length is that shotgun chambers have shallow taper angles. Ergo, a small difference in diameter shows up as a big difference in length. The good news is that, aside from very short, steeply tapered forcing cones, shotgun shells will abide a lot of "slop" in diameter and length for both over and under standard dimensions. They are not like rifle chambers in needing to be very precisely cut. A longish forcing cone, say better than 1", will take care of a slightly shortish chamber.

To put a few actual figures to R'man's excellent post, an average chamber has a taper of about .005" per inch. Present SAAMI standards I believe call for a min dia at small end of .798". Now suppose you run a solid chamber gage of .798" dia into an old 2 5/8" (2.625") chamber which just happens to have a small end of not .798" but a whole 1/1000" (.001") under size @ .797" then that gauge will stop short by .001/.005 or .200", thus showing a chamber depth of 2.425". I do not concern myself with putting up "Proper Loads" in fold crimped 2 3/4" hulls as long as the chamber is at least 2 9/16" (65mm), the normal length of most nominal British 2½" chambers incidently, with cones of at least 3/8" length. The Brits have been doing it ever since the introduction of the fold crimp incidently.
Nota Bene (Note Well), though this does not mean off the shelf "SAAMI SPEC" 2 3/4" shells, the load "Needs" to be appropriate for the gun.