Recoil

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This article is about backward force produced in firearms when fired. There are other uses of the term Recoil.

Recoil, in common everyday language, is considered the backward kick or force produced by a gun when it is fired. In more precise scientific terms, this force is equal to the derivative of the backward momentum resulting when a gun is fired.

The backward momentum is equal to the mass of the gun times its reverse velocity. This backward momentum is equal to the sums of the two forward momentums by the law of conservation of momentum and is due to and equal to the projectile's mass times its velocity added to the mass of the escaping gasses, burnt propellant, unburnt propellant, and other gasses, times their respective velocities. Provided that enough information is known for how all the velocities vary versus time, it becomes theoretically possible to determine all the accelerations versus time, and hence to calculate the total recoil force versus time. The force associated with the recoil may thus be calculated by using the law of conservation of momentum, provided that enough information regarding changes in momentum versus time is known. In practice, however, it is often easier simply to measure the recoil force resulting for a given load, rather than to estimate all the variations occurring in momentum versus time.

In most small arms, the force of the recoil is absorbed usually by the body of the shooter; in heavier guns such as mounted machine guns or cannons, the recoil is absorbed by a mounting system.

1 Recoil versus energy
2 Perception of recoil
3 Dealing with recoil in mounted guns
4 Misconceptions about recoil
5 See also
6 External links

[edit] Recoil versus energy

The energy delivered to the target of a firearm is equal to one-half of the mass of the bullet times its impact velocity squared. The energy delivered to the shooter is equal to one-half of the mass of the combined shooter and firearm, for typical firearms, times the squared velocity of the shooter and gun.

The smaller mass of the bullet, and its much higher velocity, as compared to the gun-and-shooter system, allows significantly higher energy to be imparted to the target than to the shooter. The bullet thus damages the target much more than the gun damages the shooter.

The recoil varies both between various cartridges and also within a given cartridge, based on the projectile mass, powder composition, and the velocity resulting for the bullet. The firearm can also impact the energy produced by the projectile or bullet; a longer barrel will generally provide greater velocity, and thus generate more energy, since the energy of the bullet is one-half its mass times its impact velocity squared.

See physics of firearms for a more detailed discussion.

[edit] Perception of recoil

For small arms, the way in which the shooter perceives the recoil, or kick, can have a significant impact on the shooter's experience and performance. For example, a gun that "kicks like a mule" is going to be approached with trepidation, and the shooter will anticpate the recoil and flinch in anticipation as the shot is released. This leads to the shooter jerking the trigger, rather than pulling it smoothly, and the jerking motion is almost certain to disturb the alignment of the gun and result in a miss.

This perception of recoil is related to the momentum associated with a particular gun. The total force of recoil is associated with the momentum of a gun, the momentum being the product of the mass of the gun times the reverse velocity of the gun. A heavier gun, that is a gun with more mass, will manifest the momentum by exhibiting a lessened acceleration, and, generally, result in a lessened perception of recoil.

One of the common ways of describing the felt recoil of a particular gun/cartridge combination is as "soft" or "sharp" recoiling; soft recoil is recoil spread over a longer period of time, that is at a lower acceleration, and sharp recoil is spread over a shorter period of time, that is with a higher acceleration. With the same gun and two loads with different bullet masses but the same recoil force, the load firing the heavier bullet will have the softer recoil, because the product of mass times acceleration must remain constant, and if mass goes up then acceleration must go down, to keep the product constant.

[edit] Dealing with recoil in mounted guns

A recoil system absorbs momentum, for example, by the barrel moving backwards. Cannons and such weapons without a recoil system roll several meters backwards when fired.

In a soft-recoil system, a gun's barrel is moved forward prior to shooting. As the barrel is forced backwards by the recoil force, the energy is reduced by friction, resulting in less of an overall "kick". One of the early guns to use this was the French 65 mm mle.1906; however, this method did not receive much attention until the 1970s.

Image:AT4 rocket launcher.jpg Recoilless rifles and rocket launchers exhaust gas to the rear, balancing the recoil. They are used often as light anti-tank weapons.

[edit] Misconceptions about recoil

Hollywood depictions of firearm victims being thrown through plate-glass windows are inaccurate, for if this were the case, the shooter would also be thrown backwards with equal force. Gunshot victims frequently do collapse when shot, which is usually due to the effect of the energy of the bullet acting on their central nervous systems, and is not the result of the momentum of the bullet pushing them over. Of course this does not apply if the victim is hit by heavy weapons fire, such as aircraft cannon, where the momentum effects can be significant. The recoil from such a weapon is equally enormous, and requires it to be mounted on a weapons platform.