All of the preceding. A cheap way to get bore size is to use inside calipers that extend 5 to 6 inches into the bore. Expand until you get a good feel as in using a feeler gauge, i cover the contact surface with duct tape to not scratch the bore. Measure with the Mic. and do it a couple times to make sure it is consistent. That will give your bore diameter close enough to figure the choke diameter then pattern.
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