A normal barrel would have more than .0025" deep rifling if larger than .17 calibre, but I fully understand this was merely an example, not a real barrel.

The gooove is the larger size - ie: groove depth. The bore is the tops of the lands, ie: smallest measurement inside a barrel.

You are right, we normally run bullets at groove diameter or larger when using cast bullets.

I've found some shotguns respond well to undersized balls when such ball is made to fly straight down the tube, guided by a tight fitting wad which will also 'take' the rifling and spin the ball.

Being without grease grooves, naked balls will tend to lead foul the bore. Balls that do not touch the bore do not lead foul and I've found they shoot as well or perhaps better than larger, groove diameter balls. They certainly create another option in the search for regulation of a doublegun.

In a single barreled gun, the use of groove diameter balls is not a problem, except for the leading aspect. Coating a ball with lube has solved this for shome shooters - Tinker(of this site), I think, does this for his loads.

I've even tried cloth patched round balls as used by some of my friends for moose loads with undersized balls in their duck guns. They worked perfectly, shot cleanly (killed moose) and for me, were more accurate than a ball that was almost groove diameter and in my straight rifled 13/16 bore Husky double.