Firearm propellants are a specialized type of propellant used to discharge a projectile through the barrel of a firearm. Mixtures of different chemical substances are often used to control the rate of gas release, or prevent decomposition of the propellant prior to use. Short-barrel firearms such as handguns necessitate faster-burning propellants to obtain sufficient muzzle energy, while long guns typically use slower-burning propellants. The pressure relationships between propellant chemical reactions and bullet response are described as internal ballistics.