| HS.404 | |
|---|---|
 HS.404 in theTCM-20 twin anti-aircraft configuration, displayed at the Israeli Air Force Museum | |
| Type | Aircraft cannon |
| Place of origin | Switzerland France |
| Service history | |
| Wars | |
| Production history | |
| Designer | Marc Birkigt |
| Manufacturer | Hispano-Suiza |
| Specifications | |
| Mass | 49 kg (108 lb 0 oz) |
| Length | 2.52 m (8 ft 3 in) |
| Barrel length | 1.7 m (5 ft 7 in) |
| Cartridge | 20×110mm |
| Calibre | 20 mm (0.79 in) |
| Action | Gas operated[1] |
| Rate of fire | 700-750 rounds/min |
| Muzzle velocity | 840–880 m/s (2,800–2,900 ft/s) |
| Feed system | Drum magazine, belt (later models) |
| 20 mm M1 automatic gun | |
|---|---|
| Type | Aircraft cannon |
| Place of origin | France |
| Service history | |
| In service | United States Army Air Forces |
| Used by | United Kingdom, France, Spain, US |
| Wars | |
| Production history | |
| Manufacturer | International Harvester |
| Produced | 1941 – April 1942 |
| Variants | A/N M2, A/N M3, M24 |
| Specifications | |
| Mass |
|
| Length | 2.54 m (100 in) |
| Barrel length |
|
| Cartridge | 20×110mm Hispano "A" |
| Calibre | 20 mm (0.79 in) |
| Action | Gas operated, delayed blowback[1] |
| Rate of fire | 650–700 rounds/min |
| Muzzle velocity | 870 m/s (2,900 ft/s) |
| Maximum firing range | 6,500 m (7,000 yd) |
| Feed system | 60-round M1 drum or linked belt |
| Hispano Mk.V | |
|---|---|
 20 mm Hispano-Suiza Mk V on display at theImperial War Museum Duxford | |
| Type | Aircraft cannon |
| Place of origin | United Kingdom |
| Service history | |
| Used by | United Kingdom &British Empire,Commonwealth, United States |
| Wars | |
| Specifications | |
| Mass | 38 kg (83 lb 12 oz) |
| Length | 2.184 m (7 ft 2.0 in) |
| Barrel length | 1.333 m (4 ft 4.5 in) |
| Cartridge | 20×110mm |
| Calibre | 20 mm (0.79 in) |
| Action | Gas operated, delayed blowback[1] |
| Rate of fire | 700-750 rounds/min |
| Muzzle velocity | 840 m/s (2,800 ft/s) |
| Feed system | Belt |
TheHS.404 is anautocannon originally designed by and produced by the Swiss arm of the Spanish/Swiss companyHispano-Suiza in the mid-1930s. Production was later moved to the French arm of Hispano-Suiza.
It was widely used as an aircraft, naval and land-based weapon by French, British, American and other military services, particularly duringWorld War II. The cannon is also referred to asBirkigt type 404, after its designer Marc Birkigt and later versions based on British development are known as20 mm Hispano.[2]
Firing a20 mm calibre projectile, it delivered a significant load ofexplosive from a relatively light weapon. This made it an idealanti-aircraft weapon for mounting on light vehicles, as well as afighter aircraft gun, supplementing or replacing the7.62 mm (.30 calibre) and.303 inch (7.7 mm)machine guns commonly used in military aircraft of the 1930s. The HS.404 was produced by the French subsidiary of Hispano-Suiza, and under license by a variety of companies in other countries.
The first widely used 20 mm aircraft cannon was theBecker model, introduced into German service inWorld War I. The Becker introduced theadvanced primer ignition blowback (API) design for autocannons, a concept that was quickly taken up by other companies. Notable among the resulting designs was the SwissOerlikon FF S, which was based on the Becker but introduced a number of improvements. In the 1930s,Hispano-Suiza was asked to develop a 20 mm cannon to fire through the propeller shaft (as amoteur-canon SS) of a gear-reduction inline aviation engine like theHispano-Suiza 8BeC. They took out a license on the Oerlikon FF S and made minor modifications to produce the Hispano-Suiza Automatic Cannon Type HS.7 and HS.9. 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 chief engineer of Hispano-Suiza,Marc Birkigt, began work on the design of a new weapon to replace the Oerlikon contract, based on a locking mechanism patented in 1919 by the Swedish-American machine-gun designerGustaf Swebilius [sv].[2] 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 agas-operated locking bolt.
On firing the 404, when the projectile passes a hole cut in the barrel, about halfway along its length, high-pressure gas behind the projectile is siphoned off and operates a piston that drives a rod, running along the top of the barrel, back against a cam on the bolt, unlocking it and allowing the remaining high pressure gases in the barrel to propel it and the spent cartridge backwards in a delayed sequence that allows the bolt to remain closed until the projectile has exited from the barrel. This maximizes muzzle velocity[1] and since the bolt was locked during firing, it could be lighter than that of the Oerlikon, thus facilitating an increase in rate of fire to 700 rounds per minute (rpm), a gain of about 200 rpm. In 1938, Birkigt patented it and started production in their Geneva factory.[2]
The HS.404's predecessor, theOerlikon type S auto-cannon, was rather heavy, and the movement of the heavy bolt made it best suited in static and maritime anti-aircraft defence. The lighter bolt of the HS.404 made it well suited to mounting on vehicles. The M16, an anti-aircraft version of theM3 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 theSecond World War, in theKorean War and was further developed by Israel in the post-war era.[3]
In 1938, an aircraft based version of the HS.404 was produced at the request of the French government. It was installed on a wide range of pre-war French fighter aircraft, notably in installations firing through thepropeller shaft of theHispano-Suiza 12Y engine, a system referred to as amoteur-canon (engine cannon). Due to the closed-bolt design the cannon was also suitable for synchronisation gear. The HS.404 was fed by drum magazines that could accommodate 60 (or in a fixed mount 160[4]) rounds. Since in most installations the latter was more popular, 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.[2]
Before the Second World War, the United Kingdom had embarked on a programme to develop cannon-armed fighters.[5] They acquired a licence to build the HS.404, which entered production as theHispano Mk.I intended as aeroplane armament. Its first use was in theWestland Whirlwind of 1940 and later in the more powerfulBristol Beaufighter, providing theRoyal Air Force (RAF) with powerful cannon-armed interceptors.[6] The experience of theBattle of Britain had shown the batteries of eight rifle-calibreM1919 Browning machine guns to be inadequate and prompted the adoption of autocannon armament for the primary portion of RAF fighters.[7][page needed] The Beaufighter highlighted the need for a belt feed mechanism; as anight fighter the 60-round drums needed to be replaced in the dark by theRadar/Wireless Operator, often while the aircraft was manoeuvring. The early trial installations in theHawker Hurricane andSupermarine Spitfire had shown a tendency for the gun to jam during combat manoeuvres, leading to some official doubt as to the suitability of cannon as the sole main armament. This led to theAir Ministry for a brief period specifying 12-machine gun armament for new fighters.[7]
A suitable belt-feeding system was developed byMartin-Baker and the new design was adopted by the RAF andFleet Air Arm in 1941 in a slightly modified form as theHispano Mk.II. Four cannons replaced the eight Browning machine guns in the Hurricane and in some tropical versions of the Spitfire, becoming standard armament in later fighters.[8] Most other Spitfires had only two cannons because the outboard cannon tended to freeze at high altitudes. These were complemented with four0.303 calibre (7.7 mm) or two0.50 calibre (12.7 mm) machine guns.[9][page needed]
The British were concerned their production would be inadequate and licensed production of the Hispano to the US but this production never became satisfactory and the British eventually gave up on the US versions. British production was eventually increased to the point where this was no longer necessary. The ultimate version of the British wartime Hispanos was theHispano Mk. V, which had a shorterbarrel, and lacked the cocking cylinder thus requiringmanual cocking before flight. It 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 the gun less vulnerable to freezing and mechanical stress. One of the main British fighters to use theMk. V was theHawker Tempest Mk. V Series II, which mounted two cannons in each wing.[10] Ammunition types available included semi-armour piercing, incendiary (SAPI) and high explosive, incendiary (HEI).[11] Around 42,500 Hispano cannons of various marks were manufactured byBirmingham Small Arms (BSA).
The British version was also licensed for use in the United States as theM1, with theUnited States Army Air Corps (USAAC) andU.S. Navy, which concluded that a single HS404 is analogous to three .50 machine guns in firepower while weighing less than twice as much, planning to switch to the20 mm calibre as soon as the gun could be produced in sufficient numbers. In 1941 a very large building program was established, along with the production ofammunition. When delivered, the guns proved to be extremely unreliable and suffered a considerable number of misfires due to the round being lightly struck by thefiring pin.[12] The British were interested in using this weapon to ease the demand on production in England but after receiving the M1 they were disappointed. British wing-mounted fighter weapons by this period werecocked on the ground by the aircraft armourers before flight, the pneumatic cocking mechanism used previously being regarded as unnecessary weight and detrimental to aircraft performance; 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 toyaw away from the wing with the failed gun when the guns were fired, due to the unequalrecoil, 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 shorterchamber and did not have the same problems as the U.S. version of the cannon.[12] 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 USAAF had 40 million rounds of ammunition stored but the guns remained unsuitable. The U.S. Navy had been trying to switch to using cannon on all its combat aircraft throughout the war but the conversion never occurred. As late as December 1945 theU. S. Army Chief of Ordnance was still attempting to complete additional changes to the design to allow it to enter service.[12] Some variations of the 20 mm guns used on theLockheed P-38 Lightning aircraft were produced byInternational Harvester.[13] The P-38's nose-mounted M2 featured a built-in cocking system and could simply be re-cocked in flight after a misfire, which made the misfires less of a problem than with other aircraft.[14]
The U.S. followed the British development closely and when the Mk.V was designed, the Americans followed suit with theA/N M3 but unreliability continued. After the war theUnited States Air Force (USAF) adopted a version of the M3 cannon as theM24, similar in most respects except for the use of the addition of electrical cocking, allowing the gun to re-cock over a lightly struck round.[9] The problems of the American weapons led to most U.S. fighters being equipped with theAN/M2 .50 cal Browning light-barrel machine gun throughout the war.[15]
After the war, the Hispano variants disappeared fairly quickly as fighter armament due to the introduction ofrevolver cannon, based on the prototype versions of the un-built German wartimeMauser MG 213 design. The British introduced the powerful revolving30 mmADEN cannon in most of their post-war aircraft, while the French used the similarDEFA cannon, firing similar ammunition. The USAF introduced the 20 mmM39 cannon to replace the M24, while the Navy combined the original Hispano design with a lighter round for better muzzle velocity in theColt Mk 12 cannon.[16] 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 inHonduras and is used as a light anti-aircraft gun by the army and navy in several nations.[17] The AN/M3 was developed into the Mk12 Colt 20 mm automatic cannon, one of the main weapons on boats of theMobile Riverine Force in theVietnam War and also used on some larger amphibious ships.[18]
The Hispano fired a 130 grams (4.586 oz) 20 mm diameter projectile from a 110 mm (4.331 in) long casing, the whole round weighing 257 g (9.065 oz).[19] Lengths of the projectiles varied with type but were set to variable depth in the casing to produce a total full round length of 184 mm (7.244 in) regardless of projectile type.[20] The gun had amuzzle velocity between 840 and 880 m/s (2,800 and 2,900 ft/s), depending on barrel length. Rate of fire was between 600 and 850 rounds per minute. The gun was 2.36 m (7 ft 9 in) long, weighing between 42 and 50 kg (93 and 110 lb). The British Mk V and American M3/M24 weapons were lighter and had higher rates of fire than the early HS.404 guns.[21]
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Ammunition was shipped in rectangular 10-shell fiberboard cartons. There were 12 cartons per metal-lined wooden packing crate (120 rounds).
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 six metal canisters per wooden crate (150 rounds).
