The ability of a hollow based Mini-Ball to properly engage the rifling and spin to stabilize depends on several factors. First is the composition of the projectile (what kind of lead - or the hardness); second is the thickness of the skirt and third is the widest diameter and lastly the weight. The reasons are that the skirt needs to expand to seal the bore and engage rifling, so the skirt needs to be soft enough and or thin enough to be formed when the gun goes off. The Mini-ball is slightly under bore size so either the skirt or a driving band needs to engage the rifling. Then the overall weight needs to compliment how fast it will spin as well as it ability to manage gas blow-by as it leaves the muzzle. The ability of the charge to properly flare the Mini-ball skirt and drive it forward while it is turning with the rifling and not skipping depends on a few thing. The charge has to create the right pressure, the right initial inertia (the first bump of the "bullet") and the right in-barrel velocity. The ability of the lands and grooves (rifling) to properly stabilize the Mini-ball is dependent upon its rate of twist, depth of grooves, width of grooves and contour of lands. These factors all work together. The charge moves the projectile just right that the skirt expands to seal the bore and engage rifling with a speed that does not overtake the ability to stay on the track of the rifling. The Mini-ball has enough surface area to engage the rifling, keep most of the pressure behind it and begin to make revolutions. The rifling keeps the Mini-ball centered and spinning true throughout the journey in the barrel and imparts enough spin force to stabilize the Mini-ball so it continues to rotate on the way to the target without yawing or becoming erratic. Once that happens it is the ability of the shooter that loaded properly for instantaneous discharge; controlled the trigger to reduce movement and held the sights (that are sighted in properly) until the projectile left the barrel. So, if you find just the right Mini-ball for your rifled musket, you will experience exceptional accuracy potential. If the rifling is not suitable for the projectile, the accuracy potential is limited. Some barrels (and some charges) are going to be most accurate with a patched round ball while others are best matched to a Mini-ball. It would be extremely rare and somewhat difficult to find one barrel that is equally accurate with a Mini-ball and PRB and that would take quite a bit of effort and experimentation as the charge would inevitably be different for each projectile. Some of the previous statements about Mini-balls being inherently more accurate are just not true. A round ball is not effected by the rifling if the right patch is used as the patch does the sealing and primary engaging. The round ball that is consistent cannot become unstable in flight because it doesn't have a front and back or different profiles within it. The ball has to be well balanced and is still dependent on the charge, (patch) rifling and shooter to provide its potential. With the factors being favorable, the ball can be every bit as accurate as a Mini-ball, or anything else. There is a point where the aerodynamic factor will be equated or overcome by other forces (gravity-wind-velocity) and that point will be different for a ball, a bullet or a Mini-ball.
