The main RuneScape Wiki has an article on: Cannon ammo mould
The Old School RuneScape Wiki also has an article on Cannon ammo mould.

A Cannon ammo mould is received after completing the Dwarf Cannon quest and can later be bought from Nulodion's Cannon Parts for 5 coins. It is used to make 1 multi cannon ball at a time. To make them, use a steel bar on a furnace when you have the mould in your inventory. Multi cannon balls are used as ammo for the dwarf cannon.

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