Cannonballs are ammunition used in the Dwarven multicannon. They can be made by using a steel bar on a furnace giving 25.6 Smithing experience and 4 cannonballs per bar. In order to make this item, players must have partially completed the Dwarf Cannon quest and have an ammo mould in their inventory. They require level 35 Smithing to make.
Making cannonballs is a popular method for players to train Smithing for free and reap profits. Optimal speeds would yield nearly 14,000 Smithing experience per hour from converting 540 steel bars into 2,160 cannonballs.
To smith cannonballs, have the mould in your inventory and use a steel bar on a furnace. To proceed, you can either press the space bar or click the cannonball icon. As mentioned earlier, smelting cannonballs is very slow. A full load (27 bars plus mould) takes 2 minutes and 42 seconds to complete or about two times slower than making iron bars. Doing this for money can be very useful, but it is not recommended to use for experience.
Compared to smelting iron ore with rings of forging, which disintegrate after 140 iron ores are smelted, and then forging the results, for instance, it will deliver less experience overall for the same time spent. However, it is financially rewarding even when players purchase the materials (either the iron and coal, or pre-made steel bars). Nonetheless, players are able to, at a minimum, still draw profits when they sell the cannonballs compared to breaking even or slightly profiting if the iron knife method was used instead.
It saves time if you are at a furnace that is near a bank. Edgeville furnace is the closest furnace to a bank. The second best option is Port Phasmatys while using an amulet of ghostspeak for faster banking. Port Phasmatys is accessed by emptying the Ectophial or by travelling to Port Phasmatys via charter ship. Al Kharid is another option. Use an amulet of glory to get there faster.
Although extremely slow, players can smelt one iron ore and two pieces of coal to make the steel bars and then use them to make cannonballs without returning to a bank because cannonballs are made at a furnace. Using this method, a maximum of 36 cannonballs can be made from the contents of a single inventory containing nine pieces of iron ore, 18 lumps of coal, and an ammo mould. With the addition of a coal bag from Motherlode Mine, players could increase this to 17 steel bars per load, thus yielding 68 cannonballs in a single trip. Alternatively, players could also just have the steel bars prepared, which would be significantly faster than doing it from scratch, and bring a full load of 27 with an ammo mould to make 108 cannonballs. It takes exactly six seconds to convert one steel bar into four cannonballs so smelting the bars then making the cannonballs would take 162 seconds, which is rounded up to 2.7 minutes (2 minutes and 42 seconds) per trip. Making a cannonball uses the same animation to when a player buries bones.