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Muzzle velocity is the speed of a projectile at the moment it leaves the muzzle of a 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. Some high-velocity small arms have muzzle velocities higher than the escape velocities of some Solar System bodies such as Pluto and Ceres, meaning that a bullet fired from such a gun on the surface of the body would leave its gravitational field; however no arms are known with muzzle velocities that can overcome Earth's gravity (and atmosphere) or those of the other planets or the Moon.
While traditional cartridges cannot generally achieve a Moon escape velocity (or higher) due to modern limitations of action (firearms) and gunpowder, a 1 gram (15.4324 grain) projectile was accelerated to velocities exceeding 9,000 m/s (~30,000 ft/s) at Sandia National Laboratories in 1994. The gun operated in two stages. First, burning gunpowder was used to drive a piston to pressurize hydrogen to 10,000 atm. The pressurized gas was then released to a secondary piston, which traveled forward into a shock-absorbing "pillow", transferring the energy from the piston to the projectile on the other side of the pillow.
This discovery might indicate that future projectile velocities exceeding 1,500 m/s (~4921 ft/s) have to have a charging, gas-operated action that transfers the energy, rather than a system that uses primer, gunpowder, and a fraction of the released gas. One should also note that a .22 LR cartridge is approximately three times the mass of the projectile in question. This may be another indication that future arms developments will take more interest in smaller caliber rounds, especially due to modern limitations such as metal usage, cost, and cartridge design. In a side by side comparison with the .50 BMG, the 15.4324 grain titanium round of any caliber released almost 28 times the energy of the .50 BMG, with only a 27% mean loss in momentum. Energy, in most cases, is what is lethal to the target, not momentum. 
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. Propellant quality and quantity, projectile mass, and barrel length must be balanced to achieve safety and optimal performance.
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.
Rifled barrels have twists in them that spin the football-shaped bullet so that, much like a football, it remains stable in flight. The longer the barrel the better the rotation and with that, the better the accuracy of the weapon. If you examine shot groups from a 2 inch barrel, a 4 inch barrel, and a 6 inch barrel, you’ll find tighter groups with the longer barrels.
When a bullet is fired from a handgun with a 2 inch barrel, the bullet only travels 2 inches before it leaves the barrel. Once it leaves the barrel, the gas pushing it out stops being a factor in the bullet’s propulsion. In fact, in some instances, the powder may not even be fully burned at that point. So the muzzle velocity of a 2 inch barrel is less than that of a 4 inch barrel.
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.
|Weapon||Low velocity||High velocity||Hypervelocity|
|Artillery cannons||Less than 762 m/s (2,500 ft/s)||Between 914 m/s (3,000 ft/s) and 1,067 m/s (3,500 ft/s)||Greater than 1,067 m/s (3,500 ft/s)|
|Tank cannons||-||Between 472 m/s (1,550 ft/s) and 1,021 m/s (3,350 ft/s)||Greater than 1,021 m/s (3,350 ft/s)|
|Small arms||-||Between 1,067 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)|