Are you missing the micrometer or the shotgun? If you're interested in a high tolerance measurement, be advised they will vary from gun to gun. If you just need a fairly accurate measure consider the wire method. Take four or five tight turns of wire around the area you want to measure, mark across the wire, unwind the wire, measure the distance from end mark to end mark, divide by the number of turns, divide by 3.1416 and you'll have the diameter.
