Yes you can use a roundball. Get a wooden dowel rod that is close to bore size. Insert the dowel rod from the other end of the barrel(I usually slug both ends if I have a open breech). Push the rod up to about 3/4 of an inch from the end. Put a good oil in your barrel, not too thick, but make sure you get full coverage. I prefer Kroil oil. I use a short wooden dowel to drive the lead(slug, or roundball) into the bore. I also use a wooden mallet, and I tap the lead in. You could probably pound it in, but I try to do it gently.

Then all you have to do is turn the barrel over and drive the ball and long wooden dowel out of the barrel(use the shorter rod that you drove the roundball in with), Tom.