I found this on buck and ball loads :
Experiments with buck and ball loads in fowlers and muskets have shown the to be very effective. One example was the different patterns obtained in a 20-gauge flintlock fowler. When loaded with No. 4 buckshot (Eastern, .250 diameter ball), the gun cast a scattered pattern; however, when charged with a .58 cal ball loaded first on the wadding with a charge of buckshot over the ball, the gun shot a uniform pattern at thirty yards.
The pattern rarely exceeded 24 inches in diameter, and the ball would strike within a few inches of the center of the pattern. Used against massed troops or in an ambush in bush or forest, such loads would have been devastating.
About buck & ball loads, I just did a lot of shooting with those and I used the round ball and #4 (.25 caliber) buckshot. In my 20 gauge guns I'd load the patched round ball, getting it started and just below the muzzle, then position 4 buckshot on top, add an over shot wad, and ram it down. Shooting performance was quite good.