I have bored some of them out to .62(20 ga.) They work well that way. Getting a consistent pattern with the rifling still in them is tough. If you bore out to 62 it is easier to get wads and you can use it for Turkey as well.
Where do you get "Zovac"?
The rifle is most likely a "Zouave". A rifle favored by Northern troops in the great "War against northern aggression"
I would use some type of wadding and top it off with a tight fitting cardboard patch to keep the balls from rolling out.