I don't believe that is how the gun was to be loaded originally. I would use a ball at or several thousandths over groove diameter but if I had a ball at bore diameter I'd try that too. Put in your black then an over powder card. Then a fiber or felt wad preferably soaked in Wonder lube or other black powder lubricant. Add more cards or wads to take up space. You can roll crimp or partially star crimp, then bang away.
What did the nitro loads do?
What diameter projectiles do you have?