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M40 recoilless rifle on M79 "wheelbarrow" tripod, a typical example of a recoilless rifle with a vented breech.
A recoilless rifle, recoilless launcher or recoilless gun is a type of firearm designed to eject a countermass of some type to offset the recoil of firing. The system is called a recoilless rifle if it has a rifled barrel, and a recoilless gun if it is a smoothbore.
The principle advantage of this system is that the gun assembly does not have to absorb the force of recoil itself, and so can be much lighter without the need for heavy counter-recoil systems such as hydraulic dampers. This makes them much more portable than a cannon of equivalent size, though there is a significant tradeoff in terms of muzzle velocity.
While rocket launchers without closed breeches use a recoilless operating principle, they are usually not classified as such, and the recoilless rifle/gun distinguished by the use of conventional gun propellant instead of a rocket engine. Recoilless weapons can be used to launch rocket-assisted projectiles, however, and there is some overlap between the two weapon classifications in this regard.
History[]
The first weapon to use a recoilless operating principle was the Davis gun, a 1.57-inch aircraft cannon designed by US Navy Commander Cleland Davis just prior to WW1 with the intention of attacking bombers and Zeppelins. His weapon was effectively two guns joined breech-to-breech pointing in opposite directions, with the rear-facing gun filled with grease and lead shot of the same mass as the projectile in the forward-facing gun, with the intent being for the rear-facing load to lose lethality much more quickly.
Design Details[]
There are several possible recoilless actions: the simplest is the "Davis principle" in which a second gun is joined to the first, firing a load of equal mass in the opposite direction. This system as originally designed has been historically not preferred due to the inherent danger of a gun that fires in two directions at once, as well as the difficulty of reloading the weapon and the stresses placed on it where the two guns join together.
Most recoilless weapons us a different operating principle where part of the propellant gas is vented backwards at the moment of firing, creating a forward momentum that offsets the rearward force of ejecting the projectile. While this has the advantage of not firing a physical projectile, the vented propellant gas still creates a substantial backblast which can be lethal at close range, and can also cause problems when firing the weapon in unprepared enclosed spaces.
To this end some more recent weapons use a technique pioneered by the Armbrust launcher. In this system the propellant charge is encased in a double-ended captive piston assembly, with the projectile on one side and a countermass on the other. When the propelling charge detonates, the piston expands in both directions, ejecting the shell in one direction and the countermass in the other: the countermass, however, loses energy extremely quickly due to being a highly un-aerodynamic material such as shredded plastic or salt water. The pressure in the piston assembly quickly drops to the point of being harmless.
In spite of the name, recoil forces are usually not perfectly balanced and there is generally some degree of recoil kick to a recoilless weapon.