Projectile
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A projectile is any object sent through the air by the application of some force. In a general sense, even a football or baseball may be considered a projectile, but in practical action most projectiles are designed as weapons.
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Motive force
Arrows, darts, spears, and similar weapons are fired using pure mechanical force applied by another solid object; conversely, other weapons use the compression or expansion of gases as their motive force.
Blowguns and pneumatic rifles use compressed gases, while most other guns and firearms utilize expanding gases liberated by sudden chemical reactions. Light gas guns use a combination of these mechanisms.
Railguns provide a constant acceleration along the entire length of the device, greatly increasing the muzzle velocity.
Some projectiles provide propulsion during (part of) the flight by means of a rocket engine or jet engine. In military terminology, a rocket is unguided, while a missile is guided. Note the two meanings of "rocket": an ICBM is a missile with rocket engines.
Blunt or sharp
Although blowguns use small darts, most types of guns and firearms hurl bullets, pellets, or shot made of a metal, usually lead, that are designed to deform and fragment inside a target, causing significant damage. Items like arrows, hand darts, and spears are generally tipped with sharp metallic or lithic artifacts called projectile points that allow them to more easily penetrate a target, although some types of arrows used for hunting are designed to stun or kill through shock rather than to penetrate.
Projectiles designed to be non-lethal, for example for use against riots, include rubber bullets and flexible baton rounds.
Kinetic Projectiles
Projectiles which do not contain an explosive charge are termed kinetic energy weapons or kinetic penetrators. The classic kinetic energy weapon is the bullet. Among projectiles which do not contain explosives are railguns, mass drivers, and kinetic energy penetrators, in addition to smaller weapons such as bullets. All of these weapons work by attaining a high muzzle velocity, and collide with their objective, releasing kinetic energy.
Some kinetic weapons for targeting objects in spaceflight are anti-satellite weapons and anti-ballistic missiles. Since they need to have attain a high velocity anyway, they can destroy their objective with their released kinetic energy alone; high explosive is not strictly necessary. Compare the energy of TNT, 4.6 MJ/kg, to the energy of a kinetic kill vehicle with a closing speed of 10 km/s, which is 50 MJ/kg. As such, kinetic weapons tend to be lighter than traditional explosive projectiles. However, a higher degree of precision is required in targeting for this approach.
See also hypervelocity terminal ballistics, EKV.
For a fictional kinetic weapon, see Relativistic kill vehicle.
Miscellaneous
Ballistics analyses the projectile trajectory, the forces acting upon the projectile, and the impact that a projectile has on a target. A guided missile is not called a projectile.
An explosion, whether or not by a weapon, causes debris to act as projectiles. An explosive weapon may be designed to produce shrapnel.