An interesting and quite thought provoking target is made using a full sized ball and several 'shot'. In the 4 bore, I suggest a single 1" ball plus 3 or 4 .54 cvalibre balls. You might put in normal buck, but they are tiny in comparrison to the big one. It's best to have larger 'secondary' holes. Subbing .50 cal. balls makes them at about 175gr. per 'extra' ball. .54's run around 220gr. each.