Fiftyfour: you've asked a valid and interesting question, and it would be good to listen to my answer. A muzzle loading rifle requires a ball and patch whose combined dimensions seals the bore to the bottom of the rifling, if accuracy means anything to you. Also, the patch must carry enough lubricant, having been squeezed between the lead and steel to push ALL of the fowling from the previous shot down onto the powder when you load. In order to get this phenomenon, a dead soft lead ball .005" to .010" under BORE size, and a patch between .018" and .022" is usually required to fill the rifling. That is a tight combination, and the weave of the cloth will be imprinted into the lead, as will the actual lands of the rifling. A hard ball, ie: one cast from an alloy of lead containing metals other than lead such as tin or antimony, is much more difficult to compress and will be harder to load, and worse, may cut the patch, ruining accuracy completely. So, if you have to use an alloy of lead with a hardness greater than bhn 5, you will need to use a smaller ball, ei: '010" to .015" smaller than your bore, and a thicker patch. The smooth bore and round ball is a good example of that, since there are no rifling grooves to fill. I will add that the tight combination I described above is not difficult to load with a hickory rod, but starting that combo into the muzzle will require a muzzle especially prepared, and a starter. There has been much written on this site regarding this topic, so I will not drag it out any further, but it boils down to what you want to achieve and what you're willing to do to achieve it. I personally do not even know how to spell mediocrity, and cannot tolerate it.
My .62 cal rifle has a Rice barrel whose bore is .620". My .610" Lyman ball mould casts pure lead balls that are .613". I load easily with a compressed .022" denim patch, and I use a starter. This rifle gives me and my old eyes consistent 2" groups from the bench at 100 meters. A machine rest like Hugh Toenjes' I know would produce one hole groups at that range and beyond.
My Chambers Pennsylvania fowling piece has a .620" bore as well, but no rifling. I use a .600" ball from a Lee mould and the same .022" patch, but I use pure lead, so loading is easy, and accuracy is excellent.
Sorry if I have diverged from the subject of this thread.