Back when I was shooting BPCR, I tried PP bullets in my 45-70 Pedersoli Sharps.
I was using a smooth sided (no grease grooves) 520gr soft lead round nosed bullet sized to .449. Without the patching, the bullet would just slide through the bore leaving only a few shiny spots where it came in contact with the lands. I put one wrap of paper around the bullet. This brought the OD of the bullet to about .458-.459.
I shot this over 63gr 3Fg Goex black powder and standard large rifle primers.
I don't have my notes in front of me but if memory serves me correctly, the MV was about 1150fps. Out to 500M, this was very accurate. Out of my Sharps rifle shooting prone over cross sticks with vernier type iron sights, I could hold about a 5.5" group at 500M. With a scope and shooting off a bench with sand bags, it may have done a little better.
To be honest, I switched back to regular grease groove bullets. I could hold about a 6-6.5" group with the grease groove bullets. For shooting silhouettes, that is plenty accurate enough and the grease grove bullets with a custom blend of bees wax, crisco shortening and olive oil, the fouling was much more managable. I had to swab every shot with paper patch and could get through a set (10 rounds) without swabbing in the grease groove bullets.
I know black powder is a different animal than what you're doing and the rifling in the Sharps style rifles may be a bit different. But it was a fun experiment that worked for several matches.
Jeff