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 autocannon armament for the primary portion of 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 manoeuvring. The early trial installations in the Hawker Hurricane and Supermarine 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 the Air Ministry for a brief period specifying 12-machine gun armament for new fighters. 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 guns 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 the outboard cannon tended to freeze at high altitudes. These were complemented with four 0.303 calibre (7.7mm) or two 0.50 calibre (12.7mm) machine guns. 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 the Hispano Mk. V, which had a shorter barrel, and lacked the cocking cylinder thus requiring manual 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 the Mk. V was the Hawker Tempest Mk. V Series II, which mounted two cannon in each wing. Ammunition types available included Semi-Armour Piercing, Incendiary (SAPI) and High Explosive, Incendiary (HEI). Around 42,500 Hispano cannon of various marks were manufactured by Birmingham Small Arms (BSA). US production The British version