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 bullet has exited from the barrel. This maximizes muzzle velocity 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. Anti-aircraft gun The HS.404's predecessor, the Oerlikon 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 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, in the Korean War and was further developed by Israel in the post-war era. Aircraft gun 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 the propeller shaft of the Hispano-Suiza 12Y engine, a system referred to as a moteur-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) 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. British production Before the Second World War, the United Kingdom had embarked on a programme to develop cannon-armed fighters. They 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 (RAF) with powerful cannon-armed interceptors.