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The first effective recoilless rifles (RCL) were developed during World War II as a lightweight form of anti-tank weaponry. They are capable of firing artillery-type shells at a range and velocity comparable to that of a normal light cannon, although they are typically used to fire larger shells at lower velocities and ranges. The near complete lack of recoil allows some versions to be shoulder-fired, but the majority are mounted on light tripods, and are easily man portable.
The recoilless rifle functions very much like a conventional gun. The projectile and propellant are supplied as a single round and loaded into the breech. However at firing instead of all the propellant blast driving the projectile forward a large portion is directed backwards in the opposite direction. See classical mechanics for a overview of the physics of this. Since recoil has been removed, a lot of the weighty and complex gun carriage and recoil dampening mechanism can be dispensed with.
Unlike a rocket launcher, which fires fin-stabilized rockets from a smooth bore, recoilless rifle rounds resemble conventional artillery shells. They generally have a rifling band preformed 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, or the base of the shell disintegrates.
The recoiless principle was first proved by an American, Commander Davis, during the First World War. His design had two guns connected back to back, the rear facing gun being loaded with lead balls and grease to an equal 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 aircraft. During the Second World War the Swedish company Carl Gustav developed a small 20mm device, but because Sweden was a Neutral country, it never saw action.
The first recoilless rifle to enter service was the Panzerabwehrwerfer 7,5 (or PAW), a 75 mm gun developed to give German airborne troops some useful anti-tank support before the Panzerfaust became widespread. 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. The U.S. did have a development program and it is not clear to what extend the design was copied, as their were in fact differences. Also, the Japenese had developed a portable recoiless anti-tank rifle, which they had reserverd for the defense of anticipated invasion of the mainland. However, as it was PAWs remained fairly rare during the war, but the US versions of the 75 started becoming increasingly common starting in 1945.
By the time of the Korean War recoilless rifles were found throughout the US forces. The original 75 mm and 106 mm versions had also been joined by new 57 mm and 90 mm versions. The "original" US recoilless rifles were the 57 mm and 75 mm followed by a 105 mm. The new models replacing these were the 90 mm and 106 mm. The Soviets likewise enthusiastically adopted recoilless rifle (actually recoilless "guns" as they were smoothbore) technology in the 1950s, most commonly in calibers 73 mm, 82 mm, and 110 mm (107 mm, not 110 mm).
The British, whose efforts were led by Denis Burney, inventor of the HESH round, were developing recoiless designs. Bunrey demonstrated the technique with a recoiless 4 bore shotgun, then produced many designs including a man portable 3.45" recoiless rifle pushed into experimental service in late 1945. Post-war work developed and deployed the BAT series of recoiless rifles culminating in the 120mm L6 Wombat. Lightweight 73 mm recoilless rifles are still in service in the Russian army in airborne units, and Soviet B10 82 mm heavy recoilless rifles are found quite commonly around the world in the inventories of former Soviet client states, where it is usually used as an antitank gun.
As the wire-guided missile became more and more popular in the late 1960s and early 1970s, the recoilless rifle started to disappear from the military. The last major use was the Ontos tank, 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 picked them up instead, albeit only 176 of them. They used them to great effect as a fire support vehicle during the Vietnam War. The crews continued to report 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.
Today the only remaining front-line recoilless rifle in the armies of most industrialized Western nations is the famous Carl Gustav rifle, an 84 mm man-portable anti-tank weapon. Similar in conception and use to the Bazooka, the weapon differs primarily in using rifling for stabilization rather than fins, and does not include the complex breech that is the mark of most RCL designs. 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. 106 mm recoilless rifles of US manufacture, mounted on jeeps or similar small vehicles, are very common in the armies of many poorer countries, where they serve in the role of tank destroyers.
Perhaps the oddest adaptation of a recoilless rocket was the Davy Crockett, which used a recoilless rifle to launch a tactical nuclear warhead, deployed (but of course never used) by the United States in the 1960's. It was undoubtedly the most destructive man-portable weapon ever devised.
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