You're thinking of round shot. It was made of iron because lead would deform on impact, and not have a tendency to bounce, or would expend it's energy in deformation when hitting a solid structure. Iron shot's biggest advantage was momentum. It would bounce along as basically a wrecking ball, causing massive injuries to anyone/thing caught in it's way, or impart it's energy into the masonry of a fortification or the hull of a ship. The early round shot was actually made of stone, around the 15th century or so.

There were a number of shot available for cannon, some which were made of lead, such as some grape/cannister shot, an antipersonnel round made up of much smaller balls.