I believe this is the gauge Joe was describing, I have 1 that I've used for years and it works well the only problem is that both legs are inside the bore and the depth is difficult to read. Also new is about 100.00.
Using a wall thickness gauge works except for the size of the thing, a bore gauge will work also, again not really portable. I have a set of chamber gauges, but its guess work on the taper of the chamber on each gun.
This gauge is cheap to make a 10.00 dial gauge from Harbor Freight, a 1x4x6 piece of wood and the probe. it can be a 3/8 wood dowel. I have tried it on several barrels, you can see precisely where the forcing cone starts and it is portable. Making it a little longer, the forcing cone length could be measured.
Jim A
