Old man with poor ways....
Put the bullet on a sheet of glass, not standing up.
With a good light behind it, measure with calipers from contact points on both ends. Depending upon how good your eyes are, you'll get damn close.
If the bullet touches the glass, it will touch the rifling. The crunch will add some, but the ogive of the bullet and depth of the rifling with be the determining factors. Getting that nit picky is a waste in my eyes. But what do I know.