I thought about this last night and now I understand (or think I do) that your diameter gauge is only 6" long. If so, that's why you gave the diameters 6" back from the muzzles. Wahtever... I still think that the effective constriction is the difference between the bore diameter(s) and the muzzle diameter(s), no matter how long the choke taper is/are. Patterning on paper will definitely tell you what you have. Silvers