A recoilless rifle (RCLR) or recoilless gun is a type of lightweight tube artillery that is designed to allow some of the propellant gases to escape out the rear of the weapon at the moment of ignition, creating forward thrust that counteracts some of the weapon's recoil. This allows for the elimination of much of the heavy and bulky recoiling mechanisms of a conventional cannon while still enabling the unit to fire a powerful projectile. Technically, only devices that use a rifled barrel are recoilless rifles. Smoothbore variants are recoilless guns. This distinction is often lost, and both are often called recoilless rifles. Though it is similar in form and appearance to a rocket launcher, it fires modified artillery shells, not rockets. The key difference from rocket launchers (whether man-portable or not) is that the projectile of the recoilless rifle has no propulsion of its own - once out of the rifle, it behaves as a normal artillery shell and does not accelerate further, as a missile or rocket would. Nevertheless there are also boost-after-launch rocket-propelled projectiles available for modern recoilless rifles.
Since some projectile velocity is inevitably lost to the recoil compensation backblast, recoilless rifles tend to fire a fairly heavy explosive shell with less range than traditional cannon, although with a far greater ease of transport, making them popular with paratroop, mountain warfare and special forces units where portability is of particular concern, as well as with some light infantry and infantry fire support units. Although the greatly diminished recoil allows many smaller and newer versions to be shoulder-fired by individual infantrymen, the majority of recoilless rifles in service are mounted on light tripods and intended to be carried by a small 2- or 3-man crew. The largest versions, such as the British 120mm L4 MoBAT and L6 Wombat, retain enough bulk and recoil to be restricted to a firm vehicular mount, such as on a jeep, truck or armored personnel carrier.
The typical recoilless gun functions very much like a conventional gun. The projectile and propellant are supplied as a single round and loaded into the breech. When fired, however, instead of all the propellant blast following the projectile out the barrel, a large portion is allowed to escape to the rear, gaining a rearward directed momentum which is nearly equal to the forward momentum of the projectile. This balance of momenta ensure that the momentum of the rifle/projectile/exhaust gas system is conserved without imparting much momentum (recoil) to the rifle itself. Since recoil has been mostly removed, the heavy and complex gun carriage and recoil damping mechanism can be dispensed with. Despite the name, it is rare for the forces to completely balance, and real world recoilless rifles do recoil noticeably (with varying degrees of severity). Recoilless rifles are maintenance-intensive weapons, and if the breech and gas ports are old, damaged, plugged or poorly maintained, the recoil-damping effect can be reduced or lost altogether, leading to dangerously powerful recoil. Conversely, if a projectile becomes lodged in the barrel for any reason, the entire weapon will recoil forward, in the manner of a rocket.
Unlike a rocket launcher, which fires fin-stabilized rockets from a smooth bore, recoilless rifle rounds resemble conventional artillery shells. They generally have a pre-engraved rifling band to engage the rifled launch tube, spin-stabilizing the projectile, hence the term "rifle". The "case" area of the shell can be perforated to vent the propellant gases which are then directed to the rear, as the base of the shell disintegrates.
Since venting hot gases to the rear can be dangerous in confined spaces, some recoilless guns such as the Armbrust and MATADOR use a combination of a countershot, smoothbore barrel and pistons to avoid both recoil and back blast. The fin stabilized Armbrust "cartridge" contains the propellant charge between two pistons with the warhead in front of one, facing forward, and an equal countermass of shredded plastic in front of the other piston. Upon firing, the propellant expands rapidly pushing the pistons outward. This pushes the projectile forwards towards the target and the countermass backwards providing the recoilless effect. The shredded plastic countermass is quickly slowed by air resistance and is harmless at a distance more than a few feet from the breech. The pistons jam at the ends of the barrel trapping the hot propellant gases inside. All this allows safe firing in enclosed spaces.
The first recoilless gun was developed by Commander Cleland Davis of the US Navy, just prior to the First World War. His design, named the Davis gun, connected two guns back to back, with the backwards-facing gun loaded with lead balls and grease of the same weight as the shell in the other gun. His idea was used experimentally by the British as an anti-Zeppelin and anti-submarine weapon mounted on an Handley Page O/100 bomber and intended to be installed on other aircraft.
In the Soviet Union, development of recoilless weapons ("Dinamo-Reaktivnaya Pushka" (DRP), roughly "dynamic reaction cannon") began in 1923. In the 1930s many different types of weapons were built and tested with configurations ranging from 37 mm to 305 mm. Some of the smaller examples were tested in aircraft (Grigorovich I-Z and Tupolev I-12) and saw some limited production and service, but development was abandoned around 1938. The best-known of these early recoilless rifles was the Model 1935 76 mm DRP designed by Leonid Kurchevsky. A small number of these mounted on trucks saw combat in the Winter War. Two were captured by the Finns and tested; one example was given to the Germans in 1940.
The first recoilless rifle to enter service in Germany was the 7.5 cm Leichtgeschütz 40 ("light gun" '40), a simple 75 mm smoothbore recoilless gun developed to give German airborne troops artillery and anti-tank support that could be parachuted into battle. The 75 was found to be so useful during the invasion of Crete that a larger 105 mm version was developed on the same basic pattern. Interestingly both of these weapons were loosely copied by the US Army, reversing the flow of technology that had occurred when the Germans copied the Bazooka, as their own 88 mm calibre Panzerschreck infantry anti-tank rocket system. The US did have a development program and it is not clear to what extent the design was copied, as there were in fact differences. The Japanese had also developed a portable recoilless anti-tank rifle which they had reserved for the defense of the anticipated invasion of the mainland. As it was, however, these weapons remained fairly rare during the war though the US versions of the 75 started becoming increasingly common in 1945.
During the Second World War the Swedish company Bofors Carl Gustaf developed a small 20 mm device, the 20mm m/42; the British expressed their interest in it, but by that point anti-tank rifles were already out of date.
In 1947, the US 75 mm was acquired as war surplus by the French military and mounted on a Vespa scooter. It was used by French paratroops as a mobile anti-tank and anti-fortification platform and saw service in Algeria and Indochina.
By the time of the Korean War, recoilless rifles were found throughout the US forces. The "original" US recoilless rifles were the 57mm and 75mm, followed by a 105 mm (the unsuccessful M27). Newer models replacing these were the 90 mm M67 and 106 mm M40 (which was actually 105 mm caliber but designated otherwise to prevent confusion of ammunition with the earlier model). The Soviets likewise adopted recoilless technology in the 1950s, most commonly in calibers 73 mm, 82 mm, and 107 mm .
The British, whose efforts were led by Denis Burney, inventor of the Wallbuster HESH round, also developed recoilless designs. Burney demonstrated the technique with a recoilless 4-gauge shotgun. His "Burney Gun" was developed to fire the Wallbuster shell against the Atlantic Wall defences, but was not required in the D-Day landings of 1944. He went on to produce many designs including a man-portable 3.45" (88 mm) recoilless rifle, the Ordnance, RCL, 3.45 in, pushed into experimental service in late 1945.
Two Burney guns were designed primarily as anti-tank weapons. One was 3.45 inches in calibre and could be fired off a man's shoulder or from a light tripod. The other was 3.7 inches in calibre, and was carried on a light two-wheeled mounting. The "Ordnance RCL. 3.45in MK 1" weighed 75 lb (34 kg), was 68.5in (1.74m) long, and fired an 11 lb (5 kg) wallbuster shell to 1,000 yards. No penetration figures were ever made public. but it is fairly certain that it could knock a 10 lb slab off the back of 6 in (150 mm ) of armour plate at any range it could hit. The 3.7 was a larger weapon weighing 222 lb (100 kg); it was 112 in (2.84 m) long and fired a 22.2 lb (10 kg) wallbuster to 2,000 yards; it is estimated that this could have dealt successfully with armour up to 10 in (254 mm) thick. Post-war work developed and deployed the BAT series of recoilless rifles, culminating in the 120 mm L6 Wombat ("Weapon of Magnesium, Battalion Anti-tank"). This was too large to be transported by infantry, and was usually towed by jeep. The weapon was aimed via a spotting rifle, which fired .50 BMG rounds whose trajectory matched that of the main weapon. Tracer rounds were fired first until hits were observed before firing off the main gun.
Lightweight SPG-9 73 mm and B10 82 mm heavy recoilless rifles are still in service in the Russian army in airborne units, and are found quite commonly around the world in the inventories of former Soviet client states, where they are usually used as antitank guns.
The 1965 war between India and Pakistan witnessed the personal bravery of an Indian soldier, Abdul Hamid, who was honoured with the Param Vir Chakra, India's highest military award, for knocking out seven Pakistani Patton tanks with a recoilless gun.
During the 1960s and 1970s, wire-guided missiles began to supplant recoilless rifles in the anti-tank role. The recoilless rifle started to disappear from the military except in areas such as the Arctic where battery-powered Dragons and wire-guided TOWs would fail due to extreme low temperatures. The former 6th Light Infantry Division in Alaska used the M67 in its special weapons platoons. The last major use was the M50 Ontos, which mounted six of the US 106 mm on a light (9 ton) tracked chassis first developed for use by the US Army airborne troops in 1950. However the Army considered them useless, and the Marines adopted the vehicle in a limited role. They used them to great effect as an anti-personnel fire support vehicle during the Vietnam War. The crews continued to report that the Ontos was a very effective fighting vehicle in this role, but the military brass continued to argue for heavier designs, and in 1970 the Ontos was removed from service and most were broken up. However the recoilless rifle found other roles, most notably in the India-Pakistan confrontation in Kashmir, where it was used against bunkers and as artillery in otherwise inhospitable terrain.
The Viet Minh also developed their own recoilless rifle under the direction of Tran Dai Nghia. The Vietnamese version was named SKZ or Sung khong giat (a Vietnamese translation of "recoilless rifle") and was used intensively in assaulting the French bunkers and fortified positions. The larger version of SKZ was DKZ or Phao khong giat ("recoilless artillery").
Today one of several remaining front-line recoilless rifles in the armies of industrialized nations is the famous Carl Gustav recoilless rifle, an 84 mm man-portable anti-tank weapon. First introduced in 1946, it is still in widespread use throughout the world today, and has even been re-introduced into the US Marine Corps as an anti-bunker weapon. The 84 mm (Carl Gustav recoilless rifle) can be used, along with 66 mm (AKA M72 LAW) and LAW 80 for mouse-holing whilst fighting in built-up areas (FIBUA). This is where impromptu "doors" are added to a building to gain entry, hopefully avoiding the prepared defences of the occupiers. Many nations also use a weapon related to the Carl Gustav, the one-shot AT-4 recoilless weapon.
Another recoilless rifle still in use is the italian-made 80 mm Breda Folgore, which was introduced in 1986 and built up to 2001. It is available in a shoulder-launched and in a tripod-mounted version. The use of boost-after-launch rocked-propelled projectiles gives the weapon a long effective range, compared to similar weapons.
Deployed by the United States in the 1960s, the Davy Crockett used a recoilless smooth bore gun to launch a tactical nuclear warhead.
Civilian use 
Older recoilless rifles are still used by the U.S. National Park Service and the U.S. Forest Service as a system for avalanche control. The Washington State Department of Transportation uses a 105 mm recoilless rifle for avalanche control on Interstate 90.
See also 
- Julio S.. Las Armas Modernas de Infantería, Abril de 1953
- U.S. Patent No. 2,466,714, Recoilless Firearm and Ammunition, Filing date: May 20, 1944, Inventors: William J. Kroeger, Clarence Walter Musser
- Snowbound: What to do About Avalanches, Seattle Magazine, December 2009, retrieved 2010-09-27
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