The Cannon ball is an item created and used as part of Between a Rock.... Players make it by using a gold bar on a furnace, with an ammo mould in their inventory, granting 37.5 Smithing experience. This makes only one large cannon ball, instead of the four cannonballs that are usually made from a steel bar.
The cannonball does not require 50 smithing to create despite needing 50 smithing to create the gold helmet later in the quest.
The golden cannon ball has a weight of 20 kg, compared with 0 kg (like all stackable items) of steel cannonballs. Strangely, the bar it is made from only weighs 2 kg.
This item is used to shoot through the rock; every other item bashes against the rock, but this one goes right through it as if it were not there. That is why players need to make the gold helmet later to shoot themselves into the rock to find the golden cannon ball. The first thing they will see is gold rocks, a lot for later mining.
Note that the players do not actually find the cannon ball; it will become lost within the rocks.