Those big round blunt noses on those bullets can cause a problem. What I would suggest is getting a compression die and compress the powder with that and not the bullet. Since it's a single shot, a crimp isn't really needed. I have a Pedersoli "Quigley", and size my bullets to .457, so you might want to slug your barrel to get it to fit your particular gun. A pointier style projectile may work better, I know they did with mine. And shot just about as well. I was running 70 gr. / weight Old "E" with a veggie wad and the gun shot straighter than I can. I am working with a 405 Gr. cast bullet with 62 Gr Old "E"/ weight. Shot well last summer, but it won't quit raining around here so haven't done anything with it this year. Pedersolis have good barrels, once you find what it 'likes' it will shoot well for you. Keep us posted.
KB
Note: look at my post in this column "What do ya think of this one". That's the one I'm shootin'.