Cannonball

Cannonballs are used as ammunition for the Dwarf multicannon. The cannon can hold 30 cannonballs at one time until it needs to be reloaded.

To smith cannonballs, you need the Ammo mould that you get from the quest Dwarf Cannon. If you've lost your Ammo mould somehow, Nulodion will sell you one. He can be found in the dwarf's camp near Ice Mountain, west of Edgeville.

Once you've got the Ammo mould, you'll need to have that in your inventory along with some steel bars. Head over to a furnace that's close to a bank, like in Port Phasmatys, then use your steel bars on the furnace to start smithing cannonballs. Once smithed, you will gain 25.5 Smithing experience per steel bar. Each steel bar yields 4 cannonballs.