Look into harvester sabots. You now would have what you can put in the barrel. Then pick the bullet you like. the 50 into 40 cal, .429", and 452" have all worked great. NEver needed to shim in the sabot to get the bullet to grab the sabot or nothing. Cloverleaf's at 100 yards with my T/C omega with .429" 240gr Hornady xtp's.
These guys might have it.
The Barnes Spitzer Boat Tail Muzzleloader Bullets are 100% copper bullets that deliver outstanding accuracy, penetration, and expansion.
www.muzzleloaders.com