Have you tried a Sharps type bullet? Smooth sided with a hollow base. I spent many years shooting original Sharps riles, with bullets designed from original sharps moulds. using pure lead and two wraps of high rag content paper we had excellent results for accuracy, using ffg black with a priming charge of IMR 4759. I don't off hand remember the bullet diameter to bore dia. but using the combination mentioned the bullet upset well, but keep in mind the Sharps barrel's groves were only about .003 deep
Bob