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.
