Muzzle velocity
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A gun's muzzle velocity is the speed at which the projectile leaves the muzzle of the gun. This contrasts with the terminal velocity, the speed of the projectile just before it hits its target. Muzzle velocities range from subsonic for some pistols to over 1800 meters per second for tank guns firing Kinetic energy penetrator ammunition. The latter velocity is close to the limit achievable with chemical propellants.
In conventional guns using gunpowder, 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 process is a limiting factor and a balance between propellant quality and quantity, projectile mass and barrel length must be found if both safety and optimal performance is to be found.
The velocity of a projectile is highest at the muzzle and drops off steadily due to air resistance. Generally, lighter projectiles lose velocity more rapidly than heavier ones. While a lighter projectile will move faster it will not carry more kinetic energy and if physically smaller, must carry a smaller internal explosive charge. As the gun's propellant charge is mostly burned before the projectile has moved very far down the barrel, adding length to the barrel will not indefinitely increase the muzzle velocity. Large naval guns will have length to diameter ratios of 38 to 1 to 50 to 1.
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.