A wadcutter .41 bullet might be short enough to stabilize, but then using it beyond 50 yds would be a problem, as they were not intended to be shot at longer ranges. If you can find a light weight, and hence, short .41 bullet, it might give good accuracy in that rifle.
The nice part of shooting PRB in BP rifles is that you get away from leading in the barrel. Seems to me that using a cast pistol bullet would create real problems. Usually, the rifling is too deep for even a soft bullet to expand into and seal the bore, so you have to use an over powder wad, or filler under the bullet. Then there is the issue of aligning the bullet in the barrel so it is straight. If you use a false muzzle for loading, its no problem. However, I don't think I would want to have to drag a false muzzle into the field for my reload. The next thing would be to cone the muzzle, so that the bullet is aligned and centered as it enters the bore. You would have to change the coning for a cast bullet, by elongating the seating portion of the cone, so that the entire length of the bullet would be in the muzzle before the taper would begin the centering. That would allow you to use a good steel rod, with a muzzle protector to help center the bullet as it is pushed down through the cone into the rifling. You still would want to use pure lead, and the diameter would need to be .001" over the bore diameter.
If you had access to a .40 cal. bullet, you might be able to Paper patch it, and eliminate leading that way. Again, the portion of the bullet that makes contact with the rifling would have to be short, so that the bullet would stabilize in the round ball Rate of Twist. But it could be done.
