A recoilless rifle (rifled), recoilless launcher (smoothbore), or simply recoilless gun, sometimes abbreviated to "RR" or "RCL" is a type of lightweight artillery system or man-portable launcher that is designed to eject some form of countermass such as propellant gas from the rear of the weapon at the moment of firing, creating forward thrust that counteracts most of the weapon's recoil. This allows for the elimination of much of the heavy and bulky recoil-counteracting equipment of a conventional cannon as well as a thinner-walled barrel, and thus the launch of a relatively large projectile from a platform that would not be capable of handling the weight or recoil of a conventional gun of the same size. Technically, only devices that use spin-stabilized projectiles fired from a rifled barrel are recoilless rifles, while smoothbore variants are recoilless guns. This distinction is often lost, and both are often called recoilless rifles.
An M40 recoilless rifle on its M79 "wheelbarrow" tripod
Jonga, mounted with 105 mm RCL gun which destroyed most of the tanks during the 1971 Indo-Pakistani war
1.57-inch Davis recoilless gun mounted in the nose of an F5L flying boat, with a parallel Lewis machine gun. Photo circa 1918.
M67 recoilless rifle
Recoil is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, as according to Newton's third law the force required to accelerate something will evoke an equal but opposite reactional force, which means the forward momentum gained by the projectile and exhaust gases (ejectae) will be mathematically balanced out by an equal and opposite momentum exerted back upon the gun.
An early naval cannon, which is allowed to roll backwards slightly when fired, and therefore must be tethered with strong ropes
Recoil while firing Smith & Wesson Model 500 revolver
Photograph of the kickback of a cannon, taken in Morges Castle, Switzerland
Recoilless designs allow larger and faster projectiles to be shoulder-launched.