Multi cannon balls can be made after completing the Dwarf Cannon quest. To make them, have a cannonball mould with you, and use a steel bar with a furnace. Every bar gives one cannonball. The cannonballs stack in the player inventory and are used as ammo for the Dwarf cannon. Cannonballs are 2x slower to smelt than normal bars.
Using a Steel bar on a furnace with no cannon ammo mould in the players inventory, quest progress is irrelevant
Using a Steel bar on a furnace with a cannon ammo mould in the players inventory, after Dwarf Cannon quest
- (you heat the steel bar into a liquid state)
- (and pour it into your cannon ball mould)
- (you then leave it to cool for a short while)
- (it's very heavy)
Making multi cannon balls requires a Smithing level of 30 and they give 25 experience.