For barrels with uneven number of grooves, a fixture is required to measure diameter, and there is no easy other way. I have fixtures for all calibers, of coarse, and fairly often measure for mold customers. However exact diameter isn't important, so long as the bore is straight and measurement at the rifling origin (barrel throat) is smaller than the cylinder throats. If your barrel slugs fall freely through the cylinder throats, fit to the cylinder throats and the bullets will think the barrel is perfect, if it is lapped, or if it feels smooth with the push through slugs. If you feel any jerks or tight spots with the push through slugs, lapping will correct it.
