Perry;
The actual choke dia is not as important as the amount of constriction. For this you have to know the bore size. If you are not checking a lot of bores, very accurate measurements can be made with a telescoping gage & micrometer as long as the gage has a long enough handle to get ahead of the forcing cone. I use a Starrett set with a 9" handle for this purpose all the time. If you have any machinist friends one of them may have one of these.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra