Yep that's buck shot. They were fired out of older smoothbore muskets with no rifling. The thought was that if they fired a single round ball, their accuracy was only good out to around 50-80 yards at their best. In the heat of battle when they were mostly just loading and shooting, the accuracy was more like 20 yards at best. Thus why they had the tactics of lining up and firing as one big mass...a single wall of lead was bound to hit something. So they started loading one big round ball with two bucky balls...the thought being that it turned one shot into three and thus at close range with their outdated line up and go at it tactics, the slaughter was horrendous. Typically they are seen in early war sites; the farm we were hunting had a mix from early war to late war encampments and I wandered into an early war one with the big bore guns. .69 caliber round balls and .69 caliber minie balls...those were HUGE CHUNKS of lead. The .58s are big but these are just crazy big. Evidently some of the soldiers had gotten their muskets converted over and some were still shooting the smoothbores.