A cartridge is a complete unit of ammunition. A modern metallic cartridge usually includes a case, primer, propellant, and bullet (or other projectile). When a gun is fired, the primer ignites the propellant, which burns very rapidly, forcing the bullet down the barrel of the firearm. The spent case is then ejected in some fashion, and the gun may be reloaded with another cartridge.

Among firearms enthusiasts, the performance of various cartridges is a hotly debated and often misunderstood issue. However, the science of ballistics is fairly exact, and anyone can consult the published figures for a given cartridge. Here is some of the information I have gathered during my own research.