I realize this is not the answer to the question originally asked but If you have access to a lathe turn down a length of hard wood or metal to a slightly smaller diameter than the chamber.
While this gauge you are making is in the lathe cut a very thin line at 2, 2 1/2, 2 3/4 inch etc. you will then have an inexpensive gauge that will last for years and will accurately measure chambers at a glance.
Of course these gauges are commercially avalible but here is a project a person could knock out in an evening and do multiple gauges.
While that may work satisfactorily for the individual gun that the gauge is made for, it could not be relied upon to work for all guns of the same gauge. There are enough variations in the inside diameter of doubles to make any gauge of this type not be universally accurate. If the chamber is .001" undersized, the gauge will read 1/4" short.
Lots has been written on this in the past.
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=466184&page=all http://forum.foxcollectors.com/viewtopic...er+taper#p12614 SRH
