I own, shoot and cast round balls for several muzzleloaders. I've always chosen a round ball slightly over groove diameter and drove it through the barrel after very lightly oiling the bore. Depending on the rifle I may make two slugs and drive one part way down the muzzle and another part way up the bore from the breech. I have also done as Jani menitoned and lengthened my chamber cast a couple inches. All have worked well for me.

NRA Benefactor 2008
NRA Patron 2007
NRA Endowment 1996
NRA Life 1988