we talking repro...
or are you both lucky AND loaded (moneywise)??
Yes is if the gun has the "whitworth rifling" the bore will be 6-sided instead of round with lands and grooves. this mates up with a typicaly round nosed, i think boat-tailed ("taper base" maybe), bullet with an hexagonal main body and no grease grooves. there are sources for the proper Molds to make thew bullets, Dixie Gun Works sells a swaging die that turns any .50cal bullet into a .45 cal whitworth slug. Also there are a few more traditional grease-grooved NON-hexagonal slugs that are supposed to work decently with the whitworth rifling and at least used to be labled in catalogs for use in the repros.
as for the fouling (ie hard to load) that is at least part of the reason that the whitworth never really garnered more military interest than it did, after about 3-4 shots at most you're gonna HAVE to give it at least a once over to ease loading. the "rifling" design did not allow anyplace for the fouling to go other than out the muzzle.