Hispano - Suiza HS . 404 The Hispano - Suiza HS . 404 was an autocannon widely used as both an aircraft and land weapon in the 20th century by British , American , French , and numerous other military services . The cannon is also referred to as Birkigt type 404 , after its designer . Firing a 20 mm caliber projectile , it delivered a useful load of explosive from a relatively light weapon . This made it an ideal anti - aircraft weapon for mounting on light vehicles , as well as a fighter aircraft gun replacing the multiple 7 . 62 mm ( . 30 caliber ) and . 303 inch ( 7 . 7 mm ) machine gun s commonly used in military aircraft in the 1930s . Development . From Oerlikon to Hispano . In the 1930s , Hispano Suiza S . A was licensed to build an engine drive shaft version of the Swiss Oerlikon FF S or Becker model , an advanced primer ignition blowback design , firing from an open bolt . The Oerlikon FF S was based on the Oerlikon 20 mm anti - aircraft auto - cannon . The Hispano - Suiza version was called the " Hispano - Suiza Automatic Cannon Type HS . 7 " and HS . 9 . The Swiss cannon , like all pre - war Oerlikon guns , was a blowback weapon with certain unique features : a barrel that does not recoil and a heavy breechblock which is never locked against the breech and actually moves forward when the gun is fired . Shortly after production began , the Hispano - Suiza and Oerlikon companies disagreed over patent rights and their business connection came to an end . In 1933 , the Swiss chief engineer of Hispano - Suiza , Marc Birkigt , began work on the design of an entirely new weapon to replace the Oerlikon contract , based on a locking mechanism patented in 1919 by the American machine - gun designer Carl Swebilius . The result was the Type 404 , or HS . 404 . While the HS . 404 resembled the parent Oerlikon FF S in many respects , its repeating mechanism was gas - operated . When the projectile passes a port cut into the barrel , hot gas behind the projectile is siphoned off and enters a chamber . There it pressed against a piston . The piston is connected to a rod that unlocks the bolt , allowing the gas pressure in the barrel to drive the bolt backward . Since the bolt was locked during firing , the heavy bolt of the Oerlikon could be replaced by a much lighter one , which greatly increased the rate of fire to 700 rounds per minute , about 200 rounds over the Oerlikon . In 1938 , Birkigt patented it and started production in their Geneva factory . In the comparable MG FF — a Swiss development of the Oerlikon — the mechanism remained an advanced primer ignition blowback , and lightening of the bolt was only achieved by a lighter charge , which also lowered muzzle velocity . Still , the Hispano retained its advantage in rate of fire throughout the war compared to the MG FF . Aircraft and anti - aircraft gun . The HS . 404 was widely used on pre - war French fighter plane designs , notably in installations firing through the drive shaft of the Hispano - Suiza 12Y engine , a system they referred to as a " moteur - canon " ( " engine cannon " ) . The HS . 404 was fed by drum magazines that could accommodate 60 rounds at most . Since in most installations the magazine could not be switched during flight , the small ammunition capacity was a weakness . In 1940 , Hispano - Suiza was developing a belt - feeding system , as well as derivatives of the HS . 404 in heavier calibres such as 23 mm but these projects were halted with the German occupation of France . The new auto cannon was not only considered the best aircraft cannon of its kind but was also suited for anti - aircraft use . While the predecessor Oerlikon auto - cannon , was rather heavy and the movement of the heavy bolt made it best suited in static and maritime anti - aircraft defence , the HS . 404 was well suited to mounting on vehicles . The M16 , an anti - aircraft version of the M3 Half - track , could be equipped with single or double American made copies of the Hispano - Suiza auto - cannon . This variant of the M3 Half - track was used by US and Commonwealth forces late in the Second World War and Korean War and was further developed by Israel in the post - war era . British production . In the buildup to the Second World War , the United Kingdom had embarked on a programme to develop cannon - armed fighters and acquired a licence to build the HS . 404 , which entered production as the Hispano Mk . I intended as aeroplane armament . Its first use was in the Westland Whirlwind of 1940 , and later in the more powerful Bristol Beaufighter , providing the Royal Air Force with powerful cannon - armed interceptors . The experience of the Battle of Britain had shown the batteries of eight rifle - calibre machine guns to be inadequate and prompted the adoption of auto cannon armament for the primary portion of Royal Air Force ( RAF ) fighters . The Beaufighter highlighted the need for a belt feed mechanism ; as a night fighter the 60 - round drums needed to be replaced in the dark by the Radar / Wireless Operator , often while the aircraft was manoeuvering to keep sight of its quarry . In addition , the early trial installations in the Hawker Hurricane and Supermarine Spitfire had shown a tendency for the gun to jam during combat manouvres , leading to some official doubt as to the suitability of cannons as the sole main armament . This led briefly to the Air Ministry specifying 12 - machine gun armament for new fighters . Subsequently a suitable belt - feeding system was developed by Martin - Baker and the new design was adopted by the RAF and Fleet Air Arm in 1941 in a slightly modified form as the Hispano Mk . II . Four cannons replaced the eight . 303 Browning machine gun s in the Hurricane and in some tropical versions of the Spitfire , becoming standard armament in later fighters . Most other Spitfires had only two cannons , because of technical difficulties ( i . e . , inadequate room for gun - heating capacity for the outboard cannon leading to the gun freezing at high altitudes ) , along with four 0 . 303 caliber ( 7 . 7mm ) or two 0 . 50 caliber ( 12 . 7mm ) machine guns . The British were concerned their production would be inadequate and licensed production of the Hispano to the US . However , the US production never became satisfactory and the British eventually gave up on the U . S . versions . British production was eventually increased to the point where this was no longer an issue . The ultimate version of the British wartime Hispanos was the Hispano Mk . V , which had a shorter barrel together with the deletion of the cocking cylinder thus requiring manual cocking before flight , was lighter and had a higher rate of fire ( desirable in aircraft armament ) , although at the expense of some muzzle velocity . The shorter barrel meant that the weapon could be housed within the wing of a fighter plane , reducing drag and making them less vulnerable to freezing and mechanical stress . One of the main British fighters to use the Mk . V was the Hawker Tempest Mk . V Series II , which mounted two cannons in each wing . Ammunition types available included Semi - Armour Piercing , Incendiary ( SAPI ) and High Explosive , Incendiary ( HEI ) . US production . The British version was also licensed for use in the United States as the M1 , with both the United States Army Air Corps ( USAAC ) and U . S . Navy planning to switch to the 20 mm caliber as soon as the gun could be produced in sufficient numbers . A massive building program was set up , along with production of ammunition , in 1941 . When delivered , the guns proved to be extremely unreliable and suffered a considerable number of misfires due to the round being " lightly struck " by the firing pin . The British were interested in using this weapon to ease production in England , but after receiving the M1 they were disappointed . British wing - mounted fighter weapons by this period were cocked on the ground by the aircraft armourers before flight , the built - in pneumatic cocking mechanism used previously being regarded as unnecessary weight and detrimental to aircraft performance , so any stoppage in flight made the gun unusable until it could be cleared on the ground . The misfires also had the tendency to cause aircraft with wing - mounted guns to yaw towards the wing with the failed gun when the guns were fired , due to the unequal recoil , thus throwing - off the pilot ' s aim . In April 1942 a copy of the British Mk . II was sent to the U . S . for comparison . The British version used a slightly shorter chamber and did not have the same problems as the U . S . version of the cannon . The U . S . declined to modify the chamber of their version , but nonetheless made other modifications to create the unreliable M2 . By late 1942 the USAAC had 40 million rounds of ammunition stored but the guns remained unsuitable . The U . S . Navy had been trying to go all - cannon throughout the war but the conversion never occurred . As late as December 1945 the Army ' s Chief of Ordnance was still attempting to complete additional changes to the design to allow it to enter service . Some variations of the 20 mm guns used on the Lockheed P - 38 Lightning aircraft were produced by International Harvester . The U . S . followed the British development closely and when the Mk . V was designed , the Americans followed suit with the M3 but unreliability continued . After World War II the United States Air Force ( USAF ) adopted a version of the M3 cannon as the M24 , similar in most respects except for the use of electrically primed ammunition . The problems of the American weapons led to most US fighters being equipped with the AN / M2 . 50 cal Browning light - barrel HMG throughout the war . Post - war development . After the war the Hispano variants disappeared fairly quickly as fighter armament due to the introduction of revolver cannon s , based on the German wartime Mauser MG 213 . The British introduced the powerful revolving 30 mm caliber ADEN cannon in most of their post - war designs , while the French used the very similar DEFA cannon , firing similar ammunition . Similarly , the USAF introduced the 20 mm M39 cannon to replace the M24 , while the Navy instead combined the original Hispano design with a lighter round for better muzzle velocity in the Colt Mk 12 cannon . As a ground vehicle mounted gun , either anti - aircraft or as a general - purpose autocannon , the HS . 404 lasted into the 1960s . A powered turret variant is still in production in Honduras , and is used as a light anti - aircraft gun by the army and navy in several nations . Properties . The Hispano fired a 20 mm diameter projectile from a long casing , the whole round weighing . Length of the projectiles vary with type , but are set to variable depth in the casing to produce a total full round length of regardless of projectile type . The gun has a muzzle velocity between , depending on barrel length . Rate of fire was between 600 and 850 rounds per minute . It was long , weighing between . The British Mk V and American M3 / M24 weapons were lighter and had higher rates of fire than the early HS . 404 guns . Ammunition . US . World War II . Ammunition was shipped in rectangular 10 - shell fiberboard cartons . There were 12 cartons per metal - lined wooden packing crate ( 120 rounds ) . Post - War . The M90 series of shells were ballistically matched to make it easier to use different types without losing accuracy . Ammunition was shipped in 25 - round metal canisters . There were 6 metal canisters per wooden crate ( 150 rounds ) .