For measuring chokes there are great many different styles. Unfortunately if you don't know what the bore is, the guage is useless if someone honed the bores.
Skeet's makes a nice dial indicator bore gage, that once calibrated to the ring and inserted into the muzzle all the way will give you bore and withdrawing it will give you choke. I believe Skeet's is for 12ga. and 16 ga. 100 Straight sells once from 12 ga.-28 ga. for $450.00 or I believe they will do 10 ga. -20 ga.

David