I use the one Mark cites above, the CSP. It is pricey, but works easily and accurately. I have also used inside calipers and a mike, and inside dial calipers. If using inside calipers, you need the spring type with round legs. The flat leg ones won't reach far enough into the bore. My CSP set is the short version, and reaches 9" into the bore, which is more than adequate for checking chokes.