I used to just tap a two-diameter cast bullet into the muzzle to see how it fits, then I'd do the same thing from the chamber end, only until the rifling engages the shank portion. It was handy to use a slightly larger caliber bullet for this because I wanted to see what any engraving looked like. I suppose an existing mould could be "beagled" to increase the bullet diameter for this as long as you remember that you're dealing with a slightly ovate bullet. Just tap the bullet into place and back out at both ends as fiver suggested. To measure a bore riding nose I would think a caliper would measure close enough for this purpose.