If you have a slug from the bore any good machine shop will have a micrometer that can measure that.
You can do it yourself also by wrapping a know thickness of brass shim stock around the slug and take your measurement with your standard calipers or micrometer from the measurement you get subtract the thickness of the stock times 2 and you have the dia.
It comes in handy when your measuring those five grooves S & W handgun slugs.
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