Cannonballs are stackable and are fired from a Dwarven Multi Cannon. Use them on a Dwarven Multi Cannon and a maximum of 30 cannonballs will be loaded into it. They can do up to 30 damage on an opponent and once they are fired they cannot be retrieved. Smithing 4 cannonballs yields 25.6 experience.
You must have completed the Dwarf Cannon question to be able to purchase the ammo mould from Nulodian to be able to smelt Cannonballs.