Andy, I don't know anything about the bore mike you bought and perhaps it's atypically good, but generally those sold for a wide range of shotgun bores but with a single measuring head and one calibration ring will "drift" at the high and low end of its measurement limits. Thus readings will be most accurate within a narrow range +/- of the calibration ring diameter. Otherwise it could be several thou off true size. I well remember a popular bore mike I had at one time that was .015" off true size when calibrated at .700" diameter and measuring a 10 bore. After that I bought a Stan Baker bore mike set with three measuring rods and three calibration rings.
PS: I here use the machinist word "mike" rather than "mic" as seems to have been adopted into shotgun terminology nowadays.
