If you are shooting black powder, then the bullet patched diameter can be undersize (right down to bore size) yet still slug up to fill the grooves perfectly. Once normally needs to use a lube wad on the powder (wax paper or beeswax disk separating powder from lube disk - 1/4" thick worked perfectly with 122gr. 2f in my RB with 580gr. bullet (.45 3 1/4"))

To do this, they must be very soft, no harder than 30:1 perhaps. 40:1 to pure lead, also usable or preferred.

If wanting to shoot harder bullets, they must be groove diameter or slightly exceed groove diameter.

Ahh - Huvius, you beat me to it. Good stuff.