Consider that you have two concentric circles the bore and the groove dia.
I have a set of plug/pin gauges which I use to measure the bore, but it can be done at the muzzle with calipers.. I can usually measure from a land across the bore to a groove with my dial calipers; I make this measurement for each land/groove. Average these measurements. You now have bore plus one land. Since you know the bore dimension, subtract it, multiply by two, add the bore dia back and you have the larger concentric circle. You can use a very similar approach with the slug from slugging the barrel.
Is this accurate, well it beats not having any idea.

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