The main RuneScape Wiki has an article on: Multi cannon ball
The Old School RuneScape Wiki also has an article on Multi cannon ball.

Multi cannon balls are used as stackable ammo for the dwarf cannon. They can be made one at a time by using a steel bar in a furnace while carrying the cannonball mould. Doing so requires completion of the Dwarf Cannon quest and level 35 Smithing, yielding 25 experience. Smelting the cannonball is 2x slower than smelting ores into bars.

Messages

Using a Steel bar on a furnace with no cannon ammo mould in the players inventory, quest progress is irrelevant

  • (you heat the steel bar)

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)

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