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An autocannon or automatic cannon is a rapid-fire projectile weapon firing a shell as opposed to the bullet fired by a machine gun. Autocannons often have a larger caliber than a machine gun (i.e., 20 mm or greater). Usually, autocannons are smaller than a field gun or other artillery, and are mechanically loaded for a faster rate of fire. They can use a variety of ammunition: common shells include high-explosive dual-purpose types (HEDP), any variety of armour-piercing (AP) types, such as composite rigid (APCR) or discarding sabot types (APDS).
Although capable of generating a high rate of fire, autocannons overheat if used for sustained fire, and are limited by the amount of ammunition that can be carried by the weapons systems mounting them. Both the U.S. 25 mm Bushmaster and the British 30 mm Rarden have relatively slow rates of fire so as not to use ammunition too fast. The rate of fire of a modern autocannon ranges from 90 rounds per minute (British RARDEN) to 2,500 rounds per minute (GIAT 30). Systems with multiple barrels can have rates of fire over 9000 (GSh-6-23) rounds per minute.[1] Such extremely high rates of fire are effectively employed by aircraft in air-to-air combat, where the target dwell time is short and weapons are typically operated in brief bursts.
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The Puckle gun, invented by James Puckle of London during the early 18th century, is a precursor to modern autocannon. This was a heavy tripod mounted single-barreled weapon with up to nine chambers in a cylinder. It used a crank mounted at the rear to rotate the cylinder and align the chambers with the barrel. Despite the ability to fire and reload much faster than the average soldier could manage with a flintlock musket, it failed to attain much interest among the British military or any potential investors. This gun, and similar historical weapons, may more accurately fall under the heading of volley guns or mechanical machine guns. Early attempts at rapid-firing weapons in general failed to reach widespread usage due to lack of interest as well as technological limitations, such as the difficulty in manufacturing parts to sufficiently fine tolerances. It was not until the 19th century, with the development of self-contained primers and smokeless powders that rapid-firing weapons were practical.
The first modern autocannon which made use of all of these innovations was the British QF 1 pounder, also known as the "pom-pom" gun. This was essentially an upscaled version of the Maxim gun that was the very first successful fully automatic machine gun, requiring no outside stimulus in its firing cycle other than holding the trigger. The pom-pom fired 1-pound gunpowder-filled explosive shells at rate of over 200 rounds a minute: much faster than conventional artillery while possessing a much longer range and more firepower than the infantry rifle.
During the First World War, autocannon were mostly used in the trenches as an anti-aircraft gun. The British used pom-pom guns as part of their air defenses to counter the German zeppelin airships that made regular bombing raids on London, but they were of little value, as their shells neither ignited the hydrogen of the zeppelins, nor caused sufficient loss of gas (hence lift) to bring then down. Attempts to use them in aircraft failed as the weight severely limited both speed and altitude and it was incapable of making a successful interception. The more effective QF 2 pounder naval gun would be developed during the war to serve as an anti-aircraft and close range defensive weapon for naval vessels.
Autocannon would serve in a much greater capacity during the Second World War. During the inter-war years, aircraft underwent an evolution and the all metal monoplane replaced wood and fabric biplane. The subsequent increase in speed and durability greatly reduced the window of opportunity for defence. Heavier anti-aircraft cannons had difficulty tracking fast-moving aircraft and were unable to accuratly judge altitude or distance while machine guns possessed insufficient range and firepower to bring down aircraft consistently. Weapons such as the Oerlikon 20 mm cannon and the Bofors 40 mm would see widespread use by both sides during World War II, not only in an anti-aircraft capacity, but as a weapon for use against ground targets as well. Continued ineffectiveness against aircraft despite the large numbers installed during World War 2 led, in the West, to the removal of almost all shipboard anti-aircraft weapons in the early post-war period. This was only reversed with the introduction of computer controlled systems.
The German Panzer II light tank, which was one of the most numerous tanks in German service during the invasion of Poland and the campaign in France, used a 20 mm autocannon as its main armament. Although ineffective against tank armor even during the early years of the war, the cannon was effective against light-skinned vehicles as well as infantry and also was used by armored cars. Larger examples, such as the 40mm calibre Vickers S, were mounted in ground attack aircraft to serve as an anti-tank weapon, a role to which they were suited as tank armor is often lightest on top.
In aircraft, several factors brought about the replacement of rifle-caliber machine guns by autocannon. Early autocannon were heavy and had a slow rate of fire, severely limiting the ability of the firer to use them effectively. Along with the speed, the size of aircraft grew substantially, particularly in the 1930s, so that the weight was less of an issue while the rate of fire, and the reliability were greatly improved. During World War 2, the use of armour plate protecting the pilot and other vulnerable areas caused machine gun rounds to ricochet off. Similarly self sealing fuel tanks protected against machine gun rounds, which was able to seal small holes. While a lot of machine gun rounds and a fair amount of luck were needed to cause critical damage, a single cannon shell could instantly sever essential structural elements, penetrate armour or open up a fuel tank beyond what self sealing compounds could handle, even from a fairly long range. By the end of the war, virtually all fighter aircraft mounted cannon of some sort, the only exceptions being the .50 caliber heavy machine guns used by the United States Army Air Forces.
The German Luftwaffe deployed small numbers of the experimental Bordkanone series of heavy aircraft cannon in 37, 50 and 75 mm calibres, mounted in gun pods under the fuselage or wings. The 37 mm BK 37 cannon, based on the German Army's 3.7 cm FlaK 43 anti-aircraft autocannon was mounted in pairs in underwing gun pod on a small number of specialized Stuka Panzerknacker (tank buster) aircraft. The BK 5 cm cannon, based on the 5 cm KwK 39 cannon of the Panzer III tank, was installed in Ju 88P bomber destroyers, which also used other Bordkanone models, and in the Messerschmitt 410 Hornisse bomber destroyer. 300 examples of the BK 5 cannon were built, more than all other versions. The PaK 40 semi-automatic 7.5 cm calibre anti-tank gun was the basis for the BK 7.5 in the Ju 88P-1 heavy fighter and Henschel Hs 129 twin engined ground attack aircraft.
The development of guided missiles was thought to render cannons unnecessary and a full generation of fighters was built without them, however the U.S. Air Force discovered during the Vietnam War that there was still a need for them, particularly for firing warning shots, and for targets that did not warrant the expenditure of a (much costlier) missile and so all fighters since have been so equipped.
After the Second World War, autocannon continued to serve as a versatile weapon in land, sea or air applications and continues to do so to this day. Examples of modern autocannon are the 25 mm M242 Bushmaster mounted on the M2/M3 Bradley, updated versions of the Bofors 40 mm anti-aircraft gun, the Mauser BK-27 and the 20 mm M61A1, an electrically powered gatling gun. Another role that has come into association with autocannon is that of the close-in weapon system on naval vessels, in which they are used to destroy anti-ship missiles and low flying aircraft.
The precise definition of an autocannon is often confusing, as there are other weapons which fulfil much of the criteria that define it, however a useful definition is that an autocannon is a large machine gun that fires an explosive or other-filled shell, whereas a true machine gun fires a solid bullet only. Like the machine gun, an autocannon is designed for fully automatic fire.
Another weapon that is similar to the autocannon is the automatic grenade launcher. This is usually mounted on a tripod or on a vehicle and is capable of firing explosive shells at a high rate of fire. The main item of distinction is that they fire very low velocity ammunition.
Larger forms of artillery have been fitted with automatic loading systems and may technically be considered autocannon. Several modern tanks such as the Russian T-80 and the French Leclerc use autoloaders on their 120 mm guns to reduce the crew complement from the usual four (commander, driver, gunner, and loader) down to three by eliminating the loader. These usually achieve a rate of fire similar to that of a human operator (which excludes them from the autocannon category), though future developments such as the Future Combat Systems Mounted Combat System may boast increased rates of fire. Self-propelled artillery also follow the same trends as tanks: the PzH 2000 a German self-propelled howitzer, is capable of firing at a sustained rate of 10 to 13 rounds a minute, but also in rapid fire at three rounds in nine seconds or 10 rounds in fifty-six seconds. It employs an autoloader as well as two loaders in order to achieve these rates of fire.
Modern naval guns, such as the Italian Otobreda 76 mm and American Mark 45 5"/54 gun are capable of extremely high rates of sustained fire. They use fully automatic loaders to load from a magazine, allowing them high rates of fire against surface and air targets. Automatic naval guns came into use after the decline of large naval guns and increasing use of missile armament.