Should be able to use calipers or mic and get the outside max. diameter. Measuring land height will, of course, not give a true, inner bore size - but will be fairly close, as would an inside expanding ball-gauge measurement of the gun's muzzle. Inside corner of one to the 'closest to opposite' outside corner. There must be a mathematical formula for decifering this from a single land height, using the major groove diameter produced by actual measure of a test slug.

A ball-gauge measure of the bore and outside measure of a slug were made would give both as accurately as necessary.

Actually, the major outside diameter of the slug will either be .736" or smaller.