To make cannonballs, a player must use a steel bar on a furnace (or a portable forge in "smelt" mode), while having an Ammo mould in their inventory or tool belt. This will result in production of cannonballs from the Steel bar. Each bar makes four cannonballs and yields 25.6 Smithing experience.
It is obtained during the Dwarf Cannon quest from the Dwarf engineer Nulodion. He is usually at the entrance of the Dwarven mines south of Ice Mountain and north-east of Falador. The ammo mould is priced at 5 coins, and can be bought again from Nulodion after the quest. If it is dropped and players talk to Nulodion, he will give another one for free.