My Garndfather loaded his own shells, back in the 20s-40s. Several of the shells were "pumpkin balls", single round balls. He used them as slugs. Never said anything bad about them that I remember.
I would expect them to be slightly less accurate than slugs, being round balls with no spin.
I still have a couple of these paper shells, with the ball clearly visible through a hole in the top wad. (roll crimp). My grandfather never cast his "punkin' balls", he FOUND them! There were (are) original musket balls from the French & Indian, and Revolutionary wars that he found as a child, combing the old battlefields in the area where he grew up. He also had an impressive collection of stone arrowheads that he had found during his youth.
Musket balls were pure lead, so were able to be shot through even the old full choke guns, as the soft lead would compress.
All else being equal (and it almost never is) bigger bullets tend to work better.