The first U.S. percussion rifle, namely the Model 1841 aka. the "Mississippi rifle" used by Jeferson Davis' volunteers in the Mexican War was a patched round ball shooter with seven deep rifling grooves. While the rate of twist of later rifle muskets looks like a good match with a patched round ball, the U.S. .58 cal. had three shallow rifling grooves that were configured for use with conical bullets, namely the Minié/Burton-type.
The Model 1841 was used with conical bullets too, but accuracy suffered. U.S. Army Ordnance tests determined that a rifled .69 with a conical bullet was more accurate than the 1841 with patched round ball, but that the recoil was fatiguing to the firer.The result of further testing led to the development of the .58 caliber family of rifled arms Model 1855.
As for why the mould may be single cavity, I think that this is fairly standard for larger sized bullets. There are concerns with keeping the mould itself hot enough, and that it should not cool too rapidly, since a lot of lead has to flush the interior of the mould in order to make a solid bullet. Smaller calibers can permit multiple mould chambers.
One can load round balls if s/he so desires, but there are real reasons the shallow and broad three groove barrels work better with conicals. Keep your powder dry!
