These Cannonballs are used when repairing Dwarf multicannons within the Artisans Workshop. They can be made by using a Steel ingot II on a furnace with a Cannonball mould in the inventory. Four cannonballs will be made from one ingot. Eight of these cannonballs are needed to test the repaired cannon.
This item can be taken outside of the repair area, but cannot be banked. If destroyed, you will have to make new ones. They cannot be used in place of normal cannonballs with a regular cannon, and unlike regular cannonballs are not stackable.