Naaah Mikey, that's an oversimplification. You must use the surface area of the 12 gauge and 8 gauge bores, as measured. On the measurement it's my experience that bore mikes made to measure up to 12-gauge bore diameters will "drift" and read somewhat oversize with 10-gauge bores, and no doubt an even greater drift if the mike can be expanded for 8-gauge bores. From what I understand the nominal 8-gauge bore is .835" diameter. Probably best to "slug" the barrel with a somewhat larger lead slug and measure same with a caliper or regular 1" outside mike.
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