The projectile is the component that does the work required of a firearm. Generally, a bullet is a single projectile fired from the barrel of a firearm and is part of the gas seal system. Just as the rear of the gun barrel must be sealed, the bullet forms a temporary seal at the opposite end of the cartridge case (obturation), allowing gas pressure to build to levels required for good combustion and velocity.
The earliest cannon projectiles were ball-shaped stones rounded to fit a crude cannon bore. As metal-casting technology improved, balls were cast of common metals, such as iron. Casting allowed for a more precise shape and uniformity of size and weight.
When small arms evolved, everything had to be scaled down for portability. Small balls of stone or iron were usable but lightweight, losing velocity and kinetic energy faster than heavier ones. A light projectile may have higher muzzle velocity, but the total energy deposited on target is more important than velocity. Lower velocity can be compensated for by increased mass.
A common ore, galena, could be rendered with primitive smelting equipment to produce metallic lead. Traditionally inexpensive, lead is easily worked by hammering or casting. Lead has a low melting point that does not require the high temperatures of iron smelting; lead could be melted and cast into bullets over a campfire. The high density of lead aided in retaining long-range velocity.
The round lead ball had limitations; the only way to make it heavier for greater impact was to enlarge it, requiring a larger gun barrel. The usability of a personal firearm was limited by weight and bulk. Although large-bore guns for round balls existed, mass-produced arms seldom exceeded .75 caliber (0.75 inch).