Muzzle velocity is the speed a projectile has at the moment it leaves the muzzle of the gun. Muzzle velocities range from approximately 120 m/s (390 ft/s) to 370 m/s (1,200 ft/s) in black powder muskets, to more than 1,200 m/s (3,900 ft/s) in modern rifles with high-performance cartridges such as the .220 Swift and .204 Ruger, all the way to 1,700 m/s (5,600 ft/s) for tank guns firing kinetic energy penetrator ammunition. To simulate orbital debris impacts on spacecraft, NASA launches projectiles through light-gas guns at speeds up to 8,500 m/s (28,000 ft/s). The velocity of a projectile is highest at the muzzle and drops off steadily because of air resistance. Projectiles traveling less than the speed of sound (about 340 m/s or 1115 feet/s in dry air at sea level) are subsonic, while those traveling faster are supersonic and thus can travel a substantial distance and even hit a target before a nearby observer hears the "bang" of the shot. Projectile speed through air depends on a number of factors such as barometric pressure, humidity, air temperature, and wind speed.
In conventional guns, muzzle velocity is determined by the quality (burn speed, expansion) and quantity of the propellant, the mass of the projectile, and the length of the barrel. A slower burning propellant needs a longer barrel to burn completely, but can on the other hand use a heavier projectile. A faster burning propellant may accelerate a lighter projectile to higher speeds if the same amount of propellant is used. In a gun, the pressure resulting from the combustion process is a limiting factor on projectile velocity. A balance between propellant quality and quantity, projectile mass, and barrel length must be found if both safety and optimal performance is to be achieved.
Longer barrels give the propellant force more time to work on propelling the bullet. For this reason longer barrels generally provide higher velocities, everything else being equal. As the bullet moves down the bore, however, the propellant's gas pressure behind it diminishes. Given a long enough barrel, there would eventually be a point at which friction between the bullet and the barrel, and air resistance, would equal the force of the gas pressure behind it, and from that point, the velocity of the bullet would decrease.
Large naval guns will have length to diameter ratios of 38:1 to 50:1. This length ratio maximizes the projectile velocity. There is much interest in modernizing naval weaponry by using electrically driven railguns, which overcome the limitations noted above. With railguns, a constant acceleration is provided along the entire length of the device, greatly increasing the muzzle velocity. There is also a significant advantage in not having to carry explosive propellant, and even the projectile internal charges may be eliminated due to the high velocity – the projectile becomes a strictly kinetic weapon.
The United States Army defines different categories of muzzle velocity for different types of weapons:
|Weapon||Low Velocity||High Velocity||Hypervelocity|
|Artillery cannons||Less than 762 m/s (2,500 ft/s)||Between 914.4 m/s (3,000 ft/s) and 1,066.8 m/s (3,500 ft/s)||Greater than 1,066.8 m/s (3,500 ft/s)|
|Tank cannons||-||Between 472.44 m/s (1,550.0 ft/s) and 1,021.08 m/s (3,350.0 ft/s)||Greater than 1,021.08 m/s (3,350.0 ft/s)|
|Small Arms||-||Between 1,066.8 m/s (3,500 ft/s) and 1,524 m/s (5,000 ft/s)||Greater than 1,524 m/s (5,000 ft/s)|
- "Muzzle Velocity". Retrieved 9 June 2011.
- "The Accuracy of Black Powder Muskets" (PDF). Retrieved 9 June 2011.
- "Speed of a Bullet". Retrieved 10 December 2013.
- "120mm Tank Gun KE Ammunition". Retrieved 9 June 2011.
- "Remote Hypervelocity Test Laboratory". Retrieved 29 July 2014.
- "The Rifle Barrel". Retrieved 9 June 2011.
- "Dictionary of United States Army Terms" (PDF).