The smallest measurement is the interior before rifling is cut, ie; the "bore" is the drilled then reamed size. The groove diameter is the measurement from groove to groove after they are cut.

Thus with a groove diameter of .729" (the old 'standard' 12 bore measurement for 12 balls equaling 1 pound) and .718" for the bore means with equal lands and grooves (even number0 that the rifling is .0055" deep - ie: 5 1/2 thousandth's.

I would order the mould at groove depth or .001" OVER (they shrink a bit when hardening) and use hardened balls for large game w/ antimony or tin added). The hardened balls will not hurt the rifling.

Or - you could also use an undersized ball, say .715" - standard Lyman mould) with a cloth patch which would eliminate ALL fouling buildup - never more than one shot's fouling in the bore, which would be soft due to the black power lubricant on the patch. They work.