Autocannon An autocannon or automatic cannon is a rapid - fire projectile weapon that fires armour piercing or explosive shell s , as opposed to the bullet fired by a machine gun . Autocannons often have a larger calibre than a machine gun ( e . g . , 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 US 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 ( in the case of the British RARDEN ) , to 2 , 500 rounds per minute with the GIAT 30 . Systems with multiple barrels can have rates of fire of over 10 , 000 rounds per minute , ( the Russian GSh - 6 - 23 for example ) . 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 . History . The early days . The Puckle gun , invented by James Puckle in the early 18th century , is a precursor to modern autocannon with its heavy tripod mounted single - barreled weapon , loaded with cylindrical magazines with up to eleven chambers in a cylinder . It is however , disqualified as an auto - cannon since it was manually operated , using a crank mounted at the rear to rotate the cylinder and align the chambers with the barrel . This gun , and similar early attempts at rapid - firing weapons in general , failed to reach widespread usage due to technological limitations , such as the difficulty in manufacturing parts to sufficiently fine tolerances . It was not until the 19th century , with the development of machine tools and unitary cartridges 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 " . This was essentially an upscaled version of the Maxim gun , which 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 a 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 , autocannons were mostly used in the trenches as an anti - aircraft gun . The British used pom - pom guns as part of their air defence s 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 ( and hence lift ) to bring them down . Attempts to use them in aircraft failed as the weight severely limited both speed and altitude , thus making successful interception impossible . 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 . Second World War . Autocannons 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 , pioneered as far back as the end of 1915 , almost replaced wood and fabric biplanes . 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 accurately judge altitude or distance while machine guns possessed insufficient range and firepower to bring down aircraft consistently . Weapons such as the Oerlikon 20 mm and the Bofors 40 mm would see widespread use by both sides during the second World War ; not only in an anti - aircraft role , but as a weapon for use against ground targets as well . Continued ineffectiveness against aircraft despite the large numbers installed during the second World War , 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 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 armour even during the early years of the war , the cannon was effective against light - skinned vehicles as well as infantry and was also used by armoured cars . Larger examples , such as the 40mm Vickers S , were mounted in ground attack aircraft to serve as an anti - tank weapon , a role to which they were suited as tank armour is often lightest on top . In aircraft , several factors brought about the replacement of rifle - calibre machine guns by autocannons . Early autocannons 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 reliability were greatly improved . During the second World War , the use of armour plate to protect the pilot and other vulnerable areas caused machine gun rounds to ricochet off . Similarly self sealing fuel tanks protected against machine gun rounds , which were able to close small holes securely . 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 the capacity of self - sealing compounds , even from fairly long range . By the end of the war , virtually all fighter aircraft mounted cannon of some sort , the only exception being the United States Army Air Forces favouring the " light - barrel " Browning AN / M2 . 50 calibre heavy machine gun . 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 pod s 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 pods 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 , was installed in Ju 88P bomber destroyer s , which also used other " Bordkanone " models , and in the Messerschmitt 410 " Hornisse " ( Hornet ) 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 German Mauser MK 213 was developed at the end of the second World War and is regarded as the archetypal modern revolver cannon . With multiple chambers and a single barrel , autocannons using the revolver principle can combine a very high rate of fire and high acceleration to its maximum firing rate with low weight , at cost of a reduced sustained rate of fire compared to rotary cannons . They are therefore used mainly in aircraft for AA purposes , in which a target is visible for a short period of time . The modern era . 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 , particularly for firing warning shots , and for targets that did not warrant the expenditure of a ( much costlier ) missile , as a result most 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 continue 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 gun , the Mauser BK - 27 and the 20 mm M61A1 , an electrically powered rotary cannon . 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 missile s and low flying aircraft . Definition . The precise definition of an autocannon is often confusing , as there are other weapons which fulfill much of the criteria that define it , however a useful definition is that an autocannon is a large fully automatic gun capable of firing armour piercing , explosive - filled , or other types of shell s , whereas a true machine gun fires solid bullet s only . When used on its own , the word " autocannon " indicates a single barrel weapon . When multiple rotating barrels are involved , the word " rotary " is added , and such a weapon is referred to as a " rotary autocannon " . 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 autoloader s , which are somewhat different .