Nice traditional muzzle loader there.
ANY rifling is going to have an effect on a charge of shot. 1:66 is pretty slow, but will still have SOME effect. How much, I don't know. The rifle you are looking at is intended to fire a single patched round ball, not a charge of shot.
With a patched ball you should be seeing some darned good accuracy at 100 yards. With a charge of shot, it will be less effective than a .410 shotgun. Any particular reason you would WANT to use shot in that?
What we have here is... failure- to communicate.