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interest in Oerlikons continued, the RAF never considered buying anything but Hispanos.

Initial British thoughts centred on fitting the big cannon to twin-engined fighters (the Westland Whirlwind was designed around four of them) and there was some controversy about whether single-engined aircraft should be equipped with cannon. Trial two-gun installations in both the Hurricane and Spitfire were tested in 1939, but delays involved in securing production rights and solving various technical problems (coupled with the late arrival of the Whirlwind) meant that the Hispano was not available for any significant use in the Battle of Britain. It began to be fitted to various aircraft from late 1940, starting with the Hawker Hurricane Mk IIC, the Bristol Beaufighter and the Westland Whirlwind.

It took a little time for the Hispano to be completely accepted as a replacement for the .303" - the massive weight of the cannons had a significant effect on aircraft performance - so the Hurricane Mk IIB and even the new Typhoon IA were fitted with twelve .303" Brownings instead. The .303" remained in service throughout the war, supplementing the cannon armament of Beaufighters, some versions of the DH Mosquito and most marks of Spitfire, which did not carry an all-cannon armament until very late in the war.

The Hispano was originally introduced with a sixty-round drum magazine (permitting only six seconds of fire) and with a very long barrel to suit its intended French application as a moteur-eanon, mounted within the nose of fighter aircraft and firing through the hollow propeller hub. The British never attempted to use this layout, as it was decided from the outset to fit several cannon to each aircraft for which fuselage (in twin-engined fighters) or wing mounting were more suitable.

Great difficulties with wing mounting were initially experienced in the first Spitfire installations. To minimise the aerodynamic drag of the big magazine, the guns were mounted on their sides so that the drums were largely buried in the wings. However, the guns had not been designed to fire in this way and proved very unreliable. After various confusions (including an attempt by Bristol to promote the use of their own mechanism) a belt feed, driven indirectly by the recoil movement, was developed from the French Chatellerault design. Starting in 1941, this was retrofitted to British guns in service. The slotted 'recoil reducer' (muzzle brake) fitted to drum-fed Hispanos was deleted from the belt-fed guns as they needed the recoil impulse to drive the belt feed.

Difficulties were also caused by the need to allow the gun to recoil about 20-25mm in its mounting. There were two mounting points. One was near the muzzle and was connected to the gun by the characteristic recoil return spring wrapped around the barrel. The other was by the breech and carried the stationary ammunition feed. In the gun's originally-intended application this worked well as both mountings were attached to the rigid engine block. In wing-mounted guns, the wing sections connecting the two mountings had to be particularly rigid for the gun to work properly. This was not easy to achieve and development problems were common. The Hawker Tempest mountings were found to suffer from an unacceptable lack of rigidity when tested in 1943. In the same year, the Fairey Firefly also had major problems with flexibility and resonance when firing the cannon, and even the big Blackburn Firebrand showed flexing of the main wing spar on gun firing. Fuselage-mounted Hispanos exhibited far fewer problems.

By the end of the war the earlier versions were being replaced in British service by the shorter, lighter and faster-firing Mk V. Weight was saved partly by deleting the recocking device (never used in flight) and partly by lopping 30cm from the barrel, which sacrificed some of its high muzzle velocity in the interests of compactness. Incidentally, the firm Molins demonstrated that the gun could be redesigned to achieve 1,000 rpm, but the changes to the design were substantial and not considered worth the trouble of adopting.

The lack of a British HMG was not significant as far as fighters were concerned as the Hispano proved to be a better all-round weapon, but it had serious consequences for bomber defensive armament. The Hispano was a long and heavy gun not well suited to fitting in a turret and the need for a front mounting point caused problems, although a single gun was tested in the turret of a Boulton Paul Defiant as early as December 1939. Plans to introduce 20mm turrets were slowly progressed leading

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