A groove diameter bullet is too large, imho. You won't be able to load it from the muzzle unless it is mechanically fitted first, or you use a false muzzle or bullet starter fitted onto the muzzle as with the Remington barrels of the 1800's.
As Steve stated, graduated band diameter would be best, as with the Lee R.E.A.L. bullets. Then, the conical would slug up to fit the rifling. Shallow rifling might not be best with round ball, unless very slow rate of twist. The slower to a point, the better.
A Lee style bullet with narrow bands would be easiest loading, but should have a flatter nose for hunting. If in the 400 to 450gr. range, should work on all NA game.
Shooting slugs in muzzleloading barrels never really caught on for hunting rifles (for dangerous game) as a very soft bullet of an expanding type needed to be used. Hardened round balls were found to give superior penetration compared to soft conicals of the day.
Conicals did however, become preferred when used in the early breech loaders as hardened conicals could then be used. This occurred after the actions became capable of handling heavy charges of powder combined with the conicals.