Mike:
To do the job accurately, you need an inside mic/dial indicator style tool. Brownell's has a choice. The "Skeet's" are fine for doing this job, and fairly inexpensive. Just follow the directions with the tool. You might need special attachments to do 10-bore or 28 and smaller. But it all depends. Check 'em out.
Commonly available "plug gauges" are not accurate.
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