Traditional Muzzleloader wisdom was that round balls shoot best with a slow twist in the neighborhood of 1:70. A faster twist will work ok, if you use milder loads than hunting loads. patched round balls will shoot great in fast twist barrels when the ball is moving rather slow. For instance 1:24, will work with round balls if you keep the powder down to 25 grains.
But twist is not the only factor
The difference between a fast twist round ball barrel and a fast twist for conical bullets, like inlines, is the depth of rifling. rifling has to be deeper to properly grip a patched round ball. Conical bullets have more surface area to grip the rifling and can accuately be fired out of barrels with much shallower rifling. Round ball rifling is usually .008 or more deep, which conical bullet rifling is usually around .005 or even less.
A difference is also made by the shape and width of the rifling. Some custom muzzleloader barrels have round bottom grooves and some have lands that are significantly narrower than the grooves. I have a real tack driver barrel made 30 years ago, that has oval bottom rifling and grooves that are four times wider than the lands. (imagine a rifling cutter that is shaped like an oval cut in half length wise) The rifling is actually around .0010 deep. narrow lands can work well with deeper rifling. An all lead bullet such as the Lee REAL isn't too bad to load in one of those wide groove narrow land barrels, but in a barrel with deep rifling and equal width rifling a Maxi ball can be hard to start down the barrel. The otherthing is that concical bullets have to be able to fill the rifling and prevent blow-by (hot gases and/or powder escapeing past the bullet) Blowby can melt bullets, allow pressure past the bullet, and generally ruin accuracy. So tolerances of rifling depth and width is more critical for conical bullets. Such tolerances are not as important with patched round balls because patch thickness and the soft lead projectile can be adapted more readily to those barrels.
So in general, round ball twist for a rifle should be over 1:60, however, there are many other factors. In the 1800's there were all kinds of experiments with various types of rifling. There was gain twist, where the rifling twist accelerated(got faster) toward the muzzle. There was rifling with round topped lands, grooves with convex humps sticking up in the middle of the grooves, even choked rifle barrels. The Whitworth rifle of civil war fame used a hexagonal bore with a special hexagon shaped bullet. (I have an original flint lock from the Middle East that has a septagonal bore.)
The fellow that made my target barrel made a few special off hand barrels for target shooters that were intended expressly for 25 and 50 yard offhand shooters. Imagine a 58 cal fast twist (around 1:36) barrel that was only 7/8 across the flats( very thin barrel walls) It was intended to be shot with no more than 20 -25 grains of powder. Unfortunately some stupid jerk tried 100 grains of powder with the expected resultant shrapnel. Fortunately, the gunsmith had the foresight to engrave a max load right on the barrel flats.
This is probably more information than you are looking for, but there is no one answer fits all.
