Muzzle velocity is the speed of a projectile, usually a bullet or artillery shell, as it exits the muzzle of a firearm. Muzzle velocity is affected by the projectile's mass, the barrel's length and the propellant's quantity and quality.
Usually, longer gun barrels, lighter projectiles and faster-burning propellants result in a higher muzzle velocity. A balance must be maintained between these three factors to achieve efficiency. Long barrels allow for higher air resistance and friction, while fast-burning propellants create high pressure from the combustion; both limit the projectile's speed. Muzzle velocity differs from the projectile's overall speed because the latter slows down due to air resistance.