Recoil
Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged.
However recoil only constitutes a problem in the field of artillery and firearms due to the magnitude of the forces at play.
Gun chamber pressures and projectile acceleration forces are tremendous, on the order of tens to hundreds megapascal[nb 1] and tens of thousands of times the acceleration of gravity (g's), both necessary to launch the projectile at useful velocity during the very short time (typically only a few milliseconds) it is travelling inside the barrel.
As an example, a 8 g (124 gr) bullet of 9×19mm Parabellum flying forward at 350 m/s muzzle speed generates a momentum to push a 0.8 kg pistol firing it at 3.5 m/s rearward, if unopposed by the shooter.
[nb 2] In hand-held small arms, the shooter will apply this force using their own body, resulting in a noticeable impulse commonly referred to as a "kick".
This results in the required counter-recoiling force being proportionally lower, and easily absorbed by the gun mount.
Recoil buffering allows the maximum counter-recoil force to be lowered so that strength limitations of the gun mount are not exceeded.
Modern cannons also employ muzzle brakes very effectively to redirect some of the propellant gasses rearward after projectile exit.
This provides a counter-recoiling force to the barrel, allowing the buffering system and gun mount to be more efficiently designed at even lower weight.
Propellant gases are even more tapped in recoilless guns, where much of the high pressure gas remaining in the barrel after projectile exit is vented rearward though a nozzle at the back of the chamber, creating a large counter-recoiling force sufficient to eliminate the need for heavy recoil mitigating buffers on the mount (although at the cost of a reduced muzzle velocity of the projectile).
For this reason, establishing recoil safety standards for small arms remains challenging, in spite of the straightforward physics involved.
In summation, the total momentum of the system (ammunition, gun and shooter/shooting platform)) equals zero just as it did before the trigger was pulled.
In most cases, a gun is very close to a free-recoil condition, since the recoil process generally lasts much longer than the time needed to move the ejecta down the barrel.
This is particularly true of older firearms, such as the classic Kentucky rifle, where the butt stock angles down significantly lower than the barrel, providing a pivot point about which the muzzle may rise during recoil.
[citation needed] Modern firearms, such as the M16 rifle, employ stock designs that are in direct line with the barrel, in order to minimize any rotational effects.
Before the projectile leaves the gun barrel, it obturates the bore and "plugs up" the expanding gas generated by the propellant combustion behind it.
However, when the projectile exits the barrel, this functional seal is removed and the highly energetic bore gas is suddenly free to exit the muzzle and expand in the form of a supersonic shockwave (which can be often fast enough to momentarily overtake the projectile and affect its flight dynamics), creating a phenomenon known as the muzzle blast.
The overall recoil applied to the firearm is equal and opposite to the total forward momentum of not only the projectile, but also the ejected gas.
Similarly, recoil compensators divert the gas ejecta mostly upwards to counteract the muzzle rise.
By using internal baffles, the gas is made to travel through a convoluted path before eventually released outside at the front of the suppressor, thus dissipating its energy over a larger area and a longer time.
This reduces both the intensity of the blast (thus lower loudness) and the recoil generated (as for the same impulse, force is inversely proportional to time).
The shooter may also be physically injured by firing a weapon generating recoil in excess of what the body can safely absorb or restrain; perhaps getting hit in the eye by the rifle scope, hit in the forehead by a handgun as the elbow bends under the force, or soft tissue damage to the shoulder, wrist and hand; and these results vary for individuals.
In addition, as pictured in the image, excessive recoil can create serious range safety concerns, if the shooter cannot adequately restrain the firearm in a down-range direction.
The force that the body feels, therefore, is dissipating the kinetic energy of the recoiling gun mass.
Other than employing less safe and less accurate practices, such as shooting from the hip, shoulder padding is a safe and effective mechanism that allows sharp recoiling to be lengthened into soft recoiling, as lower decelerating force is transmitted into the body over a slightly greater distance and time, and spread out over a slightly larger surface.
The following are base examples calculated through the Handloads.com free online calculator, and bullet and firearm data from respective reloading manuals (of medium/common loads) and manufacturer specifications: In addition to the overall mass of the gun, reciprocating parts of the gun will affect how the shooter perceives recoil.
Old-fashioned cannons without a recoil system roll several meters backwards when fired; systems were used to somewhat limit this movement (ropes, friction including brakes on wheels, slopes so that the recoil would force the gun uphill,...), but utterly preventing any movement would just have resulted in the mount breaking.
The modern quick-firing guns was made possible by the invention of a much more efficient device: the hydro-pneumatic recoil system.
In this system, the barrel is mounted on rails on which it can recoil to the rear, and the recoil is taken up by a cylinder which is similar in operation to an automotive gas-charged shock absorber, and is commonly visible as a cylinder shorter and smaller than the barrel mounted parallel to it.
One of the early guns to use this system was the French 65 mm mle.1906; it was also used by the World War II British PIAT man-portable anti-tank weapon.
