A round shot is the archetypal cannonball fired by a cannon. A solid ball, its weight defined the common sizes of cannon, the 9pdr, 18pdr, 24pdr and 32pdr.Its main use was punching through the sides of other ships, sinking them or producing a spray of splinters which caused nasty wounds. Round shot were sometimes fired downwards so they bounced off the sea into the other ship.
Shore batteries could use furnaces to produce red hot heated shot, which was very effective at setting ships alight if the ball lodged in the timbers. Furnaces were considered too dangerous to use on ships
- Round shot at Wikipedia