To measure bore diameter with a Skeet's, a Manson and a vernier caliper, mark a place on the barrel near the extent of the measuring devices. Measure wall thickness, top and bottom at this point. Use a micrometer or a caliper to measure the outside diameter of the barrel at the same place, Subtract the two wall thicknesses from the outside diameter. This is your bore diameter. Now measure the choke diameters with the Skeet's gauge. Subtract the choke diameter from the bore diameter and now you have the choke constriction. You can do the same with an eight gauge and the same tools.
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