Mine is an Armisport with really, really shallow rifling even for a .69 minie musket. So far I've had to use a 14 ga. fiber wad under the patched ball to get any kind of grouping. In the for what it's worth department, I'm immersing the fiber wads in melted beeswax based lube until they stop bubbling. The round ball is 0.683" diameter from a mystery mold marked .680 that I picked up somewhere or the other. I'm not loading a tight patch with this combo, just enough to keep the ball centered up in the bore. I don't know if the ball's girth is increasing any when the wads slams it from behind. I really do think that the wad is conforming to the hind end of the ball and thus moving towards the barrel wall, in essence taking up the slack and helping the ball rotate rather than bounce down the barrel in addition to forming a gas dam.
I should figure out giving the ball a soft enough landing to see if rifling marks indicate that it's girth is increasing while the wad overcomes it's inertia.
By the way, I'm shooting stout loads. Don't know how the fiber wads might behave with plinking loads.