Muzzle velocity is the speed of a projectile at the moment it leaves the end of a firearm (i.e. the muzzle). 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 (1,100 ft/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 (approximately 2,300 m/s (7,500 ft/s)) or higher due to modern limitations of action (firearms) and gunpowder, a 1 gram (15.4324 grains) 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 (4,900 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 gr (1 g) 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 quantity of the propellant, its quality (in terms of chemical burn speed and expansion), the mass of the projectile, and the length of the barrel. A slower-burning propellant needs a longer barrel to finish its burn before leaving, but conversely can use a heavier projectile. This is a mathematical tradeoff. A faster-burning propellant may accelerate a lighter projectile to higher speeds if the same amount of propellant is used. Within a gun, the gaseous pressure created as a result of the combustion process is a limiting factor on projectile velocity. Consequently, propellant quality and quantity, projectile mass, and barrel length must all be balanced to achieve safety and to optimize 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 spiral twists carved inside them that spin the bullet so that it remains stable in flight, in the same way an American football thrown in a spiral will fly in a straight, stable manner. This mechanism is known as rifling. Longer barrels provide more opportunity to rotate the bullet before it leaves the gun. As such, longer barrels increase the overall precision of the weapon. If one examines shot groups on a paper target from a 2-inch (51 mm) barrel, a 4-inch (100 mm) barrel, and a 6-inch (150 mm) barrel, one will observe how the longer barrels produce "tighter" grouping, with bullets landing closer together on the target.
A bullet, while moving through its barrel, is being pushed forward by the gas expanding behind it. This gas was created when the trigger was pulled, causing the firing pin to strike the primer, which in turn ignited the solid propellant packed inside the bullet cartridge, making it combust while situated in the chamber. Once it leaves the barrel, the force of the expanding gas ceases to propel the bullet forth. When a bullet is fired from a handgun with a 2-inch (51 mm) barrel, the bullet only has a 2-inch (51 mm) "runway" to be spun before it leaves the barrel. Likewise, it has only a 2-inch (51 mm) space in which to accelerate before it must fly without any additional force behind it. In some instances, the powder may not have even been fully burned in guns with short barrels. So, the muzzle velocity of a 2-inch (51 mm) barrel is less than that of a 4-inch (100 mm) barrel, which is less than that of a 6-inch (150 mm) barrel.
Large naval guns will have high length-to-diameter ratios, ranging between 38:1 to 50:1. This length ratio maximizes the projectile velocity. There is much interest in modernizing naval weaponry by using electrically powered railguns, which shoot projectiles using an electromagnetic pulse. These overcome the limitations noted above. With these railguns, a constant acceleration is provided along the entire length of the device by means of the electromagnetic pulse. This greatly increases the muzzle velocity. Another significant advantage of railguns is not requiring explosive propellant. The result of this is that a ship will not need to transport propellant and that a land-station will not have to maintain an inventory of it either. Explosive propellant, stored in large quantities, is susceptible to explosion. While this can be mitigated with safety precautions, railguns eschew the need for such measures altogether. Even the projectile internal charges may be eliminated due to the already high velocity. This means 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)|