When determining what rate of twist you need for a particular usage you need to determine, within a small range, what the powder charge will be. Stability is ultimately determined by rpms, not twist rate alone. The heavier the powder charge the higher the velocity, and the higher the velocity the lower twist rate that seems to be needed to stabilize the projectile.
It is tough to find a twist rate that is fast enough for a bullet but slow enough for round balls. Thompson Center tried this decades ago, and sold many thousands of rifles with a 44" twist, AIR. They never won many serious round ball matches, but would shoot decently enough to keep some people happy, with the right charge. They also shot a bullet to minute of lung. Good enough for many deer hunters, but not for serious competition.
IDK how well gain twist would work for a bullet/ball rifle, but it's an interesting concept, and one I've never heard of being tested. My guess would be the shorter the bullet the better it would stabilize it, as with most mid-range twist barrels. Back when I competed with round ball rifles Douglas XX was the barrel to beat, with custom barrels by Ken Bresein and Jim Goodein leading the pack of custom makers.
My best ever barrel is a .45 Douglas XX with a 66" twist, 42" long and 1 1/8" across the flats. I have shot 5/8", 5 shot, 100 yard groups with it with a tight fitting ball/patch combination.
Good luck ...... SRH
