BSS,
The photos below are what I use to measures bores, chokes and chamber lengths. An inside caliper and a 1" micrometer. If you're careful they can be damned accurate and give you repeatable measurements.


Measuring chamber length, put in past the forcing cone to measure bore diameter, then do the same at the muzzle.


Using a micrometer to measure caliper for bore or choke diameter.

Steve

Approach life like you do a yellow light - RUN IT! (Gail T.)