A certain amount of energy is required to push a bullet out of the cartridge case, through the barrel of the gun and out to the target. This energy is created from the gunpowder stored inside the cartridge case.
Gunpowder is a chemical compound that, when ignited by a spark, burns very rapidly and produces a relatively large volume of hot, expanding gas. It is this expanding gas that pushes the bullet down the gun barrel and out the muzzle. The earliest form of gunpowder is called black powder and it was invented by the Chinese around 1,000 AD. Black powder is still used today in muzzle loading firearms. Black powder is quite dirty to shoot, and is also corrosive. Black powder firearms have to be cleaned at the end of each shooting session to avoid damage to the bore (the inside of the barrel).
Modern gunpowder is classified as a propellant and is much less corrosive, much cleaner than black powder and burns with much less smoke than black powder. However, guns that shoot modern smokeless powder still need to be cleaned after shooting of the burned gunpowder residue that builds up on the firearms. Today, almost all cartridges are loaded with modern, smokeless gunpowder. Modern gunpowder was first developed in the second half of the 1800s.
The fourth and final component of a round of modern cartridge ammunition is the Primer. The primer is a pressure-sensitive chemical compound that provides the spark that starts the gunpowder burning. The primer compound is similar to a child’s toy cap pistol. When the cap is struck, it goes bang with a tiny flash of flame. The primer in a cartridge acts the same way when the hammer or firing pin of the gun strikes it and produces a tiny flash of flame that ignites the gunpowder inside the cartridge case and pushes the bullet down the barrel and out to the target.