Bullet cartridges are designed to be safe until the moment when you fire them. When you pull the trigger of a gun, a spring mechanism hammers a metal firing pin into the back end of the cartridge, igniting the small explosive charge in the primer. The primer then ignites the propellant, the main explosive that occupies about two thirds of a typical cartridge’s volume. As the propellant chemicals burn, they generate lots of gas very quickly. The sudden, high pressure of the gas splits the bullet from the end of the cartridge, forcing it down the gun barrel at extremely high speed. It’s only the bullet that fires from the gun; the rest of the cartridge stays where it is. It has to be ejected after firing to make way for the next cartridge and the next shot.