Just for the fun of it I would prefer to load those round balls into cases you have charged with about 2 grains of powder, just enough to get them out the barrel and into a wad of old towels or something and measure them after. I just hate having to drive a round ball down a barrel with a 7/16 wooden dowel.
Either way you should get some decent rifling marks on the round ball and be able to get a decent measure.
I think you can also try some slightly oversized soft slugs, over just enough powder to get it out the barrel, and they might give you a good measure.