Bullet or Ball
My .54 T/C Hawken has 1:48 twist. It shoots great with either ball or PB.
The .50 CVA Mt. Rifle has 1:66 twist and the PB shoot just as good as the round ball. My CVA Mt. Stalker in .54 has a 1:38 twist. It shoots either ball or bullet just fine. I've not noticed much problems with twist until I started playing around with pistol length barrels. Most of these like the faster twist such as 1:16 and that's even with the round ball. I come to think pressure plays as much a factor as twist rate.