An autocannon is a rapid-fire projectile weapon firing a shell as opposed to the bullet fired by a machine gun. Autocannon 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 1,800 rounds per minute (Mauser BK-27). Systems with multiple barrels can have rates of fire of several thousand rounds per minute. 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.
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 of maintenance and repair. 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 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 would not penetrate or detonate upon contacting the soft surface of the zeppelins. Attempts were made to use them in aircraft though with limited success. 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 from the biplane into the monoplane configuration. Metallic alloys were used to cover the aircraft as opposed to canvas, and the cockpit was enclosed by glass. The subsequent increase in speed and durability greatly reduced the window of opportunity for attack. Heavier anti-aircraft cannons were unable to track faster-moving aircraft at lower altitudes 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, in not only anti-aircraft capacity, but as a weapon for use against ground targets as well.
The German Panzer II light tank, which was one of the most numerous tanks in German service during the Invasion of Poland and the Battle of France, used a 20 mm autocannon as its main armament. Although ineffective against tank armor even during the early years of the war, it was capable of penetrating their rear armor at close ranges. Autocannon were effective weapons against light-skinned vehicles as well as infantry, and found use in armored cars against these targets. Larger examples, such as the 40mm calibre Vickers S, were mounted in ground attack aircraft to serve as an anti-tank weapon, a role which they were effective at as the top portion of a tank's armor is usually the least armored.
In aircraft, autocannon eventually came to replace the preceding rifle-caliber machine guns. Whereas machine guns needed to strike at critical areas of an aircraft to effectively bring it down, such as the fuel tanks or cockpit, an autocannon could strike anywhere and cause enough structural damage to render an aircraft inoperable. By the end of the war, virtually all fighter aircraft mounted cannon of some sort, the only exceptions being heavy machine guns of greater than 12 mm caliber.
The German Luftwaffe did some active experimentation, and limited deployment, of their Bordkanone series of heavy calibre aircraft cannon in 37, 50 and 75mm calibres, which were often mounted in gun pods under the fuselage or wings. The BK 37 cannon, of 37mm calibre and based on the 3.7 cm FlaK 43 anti-aircraft autocannon used by the German Army, was most often found mounted in underwing gun pods, two per aircraft, of a small number of specialized Ju 87G Stuka Panzerknacker (tank buster) aircraft. The BK 5 50mm cannon, based on the design of the 5 cm KwK 39 50mm cannon of the Panzer III tank, saw installation in one model of the specialized Ju 88P bomber destroyer heavy fighters, of which the other two models of the Ju 88P designed used the two other Bordkanone models, and on the Me 410 A-1/U4 bomber destroyer version of the Hornisse heavy fighter. 300 examples of the BK 5 cannon are known to have been built, more than the other two versions. The BK 7,5 version of 75mm calibre, based on the PaK 40 semi-automatic anti-tank gun, only saw fitment in the Ju 88P-1 heavy fighter and Hs 129 B-3 twin engined ground attack aircraft. The BK 7,5 was the most massive forward-firing autocannon installation ever placed on any combat aircraft, up until the service introduction of the U.S. Air Force's A-10 Thunderbolt II ground attack aircraft, and its GAU-8 Avenger Gatling cannon in 1977.
After the Second World War, autocannon continued to serve as a versatile weapon in land, sea or air applications 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.
Machine guns for example, are also weapons which use some form of automatic action to function with little human interaction. However, these are often much smaller weapons, usually capable of being carried by hand and fired while autocannons require some kind of carriage or mounting to absorb recoil. Autocannon shells are usually of 20 mm caliber or larger, and can be explosive. Machine gun ammunition on the other hand, is usually solid shot, and lack any kind of explosive ability. In addition, autocannon usually possess much greater range and penetration capability compared to machine guns.
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 items of distinction are that they too, are usually much smaller, and fire much lower velocity ammunition with a much more limited selection.
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 13 rounds a minute, but also in rapid fire at 3 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.
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- Department of the Army. Ballistic Data Performance of Ammunition, TM 9-1907. Washington, D.C.: Government Printing Office, 1948.
- Williams, Anthony G. Rapid Fire. Shrewsbury: Airlife Publishing Ltd., 2000. ISBN 1-84037-435-7
- Alvi, M. A., Rahman, Abdur, Fathullah Shirazi: A Sixteenth Century Indian Scientist, National Commission for the Compilation of History of Sciences of India, National Institute of Sciences of India, 1968
- The GSh-6-30K, a six-barreled Russian Gatling gun, has a ROF of 6,000 rounds per minute. Williams, p. 241.