Autocannone

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 exploded by impact on soft-bodied airships, 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, it was effective weapons against light-skinned vehicles as well as infantry, and found use in 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 which they were effective at as tank armor is usually the thinnest on top.

In aircraft, autocannon eventually came to replace 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 .50 caliber used by the United States Army Air Forces.

The German Luftwaffe did some active experimentation, and limited deployment, of their Bordkanone series of heavy aircraft cannon in 37, 50 and 75 mm calibres, which were often mounted in gun pods under the fuselage or wings. The BK 37 cannon, of 37 mm 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 cm cannon, based on the design of the 5 cm KwK 39 cannon of the Panzer III tank, saw installation in one model of the specialized Ju 88P bomber destroyers, 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 cm version, based on the PaK 40 semi-automatic 7.5 cm calibre anti-tank gun, was only installed 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 autocannon installation on any combat aircraft until 1977, when the U.S. Air Force's introduced A-10 Thunderbolt II with its GAU-8 Avenger Gatling cannon.

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.