Now you made me rethink things. Have used cast HB .50cal. minis., paper patched to .513" in a 50/70 with good accuracy, but the BP fouling would build up so quickly, spent more time cleaning than shooting. One trial with a lube "cookie" under the bullet did cure the fouling problem, but accuracy was poor no matter what I did...would guess that the lube was sometimes plugging the HB and sometimes not, wich gave the variation on the target.
Would guess that with a solid base bullet, if you can get the bullet diameter to where it is just lightly engraved when loading (jut slight impressions on the paper patch...hopefully even) that the bullet would be dead center of the bore, and more likely to expand at firing more evenly. Worth a try, and I will next time out.
Haven't tried paper patched muzzle loading bullets, other than a cross patch in a friend's bench gun. That one is fun, but at 22 pounds, hardly field friendly.