Imagine you're at sea, an enemy ship bearing down on you. You have a limited amount of gunpowder, so you use it to fire one cannonball toward the ship. It hits--ping!--and sinks the enemy. You've used all your gunpowder, but it was worth it. This is a simple example of the concept of cannonball stacking, which is why we make bullets (low-cost, low-risk, low-distraction experiments) before firing the cannonball (concentrating resources into a big bet).
In the case of an actual cannonball, the "bet" is not about whether the shot will hit, but how much damage it will cause. A cannonball's size, material, and shape all contribute to its ability to sink a ship. The shape of the cannonball is determined by the size of its barrel, and its surface is made from a hard material such as stone, metal, or even glass.
The larger the cannonball, the more likely it is to have telltale "remnants" or projections that help in its authentication. These projections occur along the mold seam of a solid shot casting and may be either an "equator" or "poles" line.
For medium and large cannonballs, a telltale equator or poles line is not always present; however, for these sizes the diameter of a cannonball may be measured with calipers or a machinist's feeler gage in the plane of the "poles" and in the opposite red arrow lines shown below. This will allow the measurement of any cast metal surface projections on a cannonball that could be damaging to the cannon bore upon firing.