"A friend, who shoots much more black powder that I do, mentioned that the issue with harder lead and round balls was that of getting the lead to engrave in the rifling. His solution was either to use soft lead (his preference) and thin patches or harder lead with thicker patches and HOPE that the rifling would still spin the ball."
There is another one that doesn't make a lot of sense to me. WW balls will show the pattern of the fabric weave when started. If you are having trouble starting them, your ball is too large. The ball is not, or should not, be engraving the rifling. That is the job of the patch, to hold the grooves. You'll never see a short starter in an old original bag, nor any in the accouterments of anyone who has shot a long time.
When developing a load for a round ball rifle, the patching should always be selected before the ball diameter. The striped pillow ticking has been the standard for many decades. The precut, prelubed patches available are usually too thin, with a low thread count. Not what you want. They're a sucker product.
The rifleing will spin the ball, regardless of the alloy or straight lead. As the mass of the projectile starts up the bore incline, it is supported by the patch riding in the grooves, and there is no way the ball is going to spin at a different rate than the patch. If a ball is recovered showing any marked signs of obduration from a rifle, I strongly suspect the ball is not being seated correctly on the charge.