My telescoping gage set I mentioned has a small head that goes down to .375", small enough to measure even the choke on a .410. 8-Bore's small hole gage does the same thing, both are fit to the bore, extracted & measured with a micrometer. Unless you are in business & need to measure a lot of bores in a short time, you don't need a lot of expensive equipment to measure an occasional bore. You are only measuring a diameter here, concentricity plays no part as it does in measuring wall thickness.

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