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Hawk, Bell P-39 Airacobra and early Curtiss P-40 Tomahawks these were fuselage-mounted. These synchronised installations were criticised by British testers as they reduced the rate of fire to 400-500 rpm. As the number increased, guns were moved to the wings and synchronised weapons were abandoned, except in the specialised P-39 and Bell P-63 Kingcobra which were specifically designed for a heavy nose-mounted armament. While the Browning s projectiles lacked the destructive effect of cannon shells, their fairly high velocity and combined rate of fire made a six-gun installation a formidable armament, with the added advantage over mixed-gun installations of a greater ease of aiming.

Contrary to the general impression, the Browning M2 was not the only aircraft gun tried or adopted by the USA. The USAAF, not concerned with shooting down bombers, were not at first particularly interested in adopting 20mm cannon but they did expend much effort in achieving higher muzzle velocities in HMG calibres, partly by using lighter bullets and partly by developing weapons to fire larger-capacity cartridges. One of the best known of these is the T17 series, essentially a re-engineered MG 151 designed around the very powerful .60" cartridge originally intended for an abortive anti-tank rifle. This was also necked down to .50" in search of higher velocity (up to 1,340 m/s being achieved with a 32.4g projectile), as was the 20mm Hispano case, but none of these saw service, although over 300 T17s were built and more than six million rounds of .60" ammunition produced.

The 20mm HS 404 was standardised by the USA in 1940 and manufactured under contract by Bendix, and subsequently other companies, mainly in the AN-M2 version (the designation indicating adoption for both the army and navy), but saw relatively little use. Only the Lockheed P-38 Lightning (which in most versions carried one), the Northrop P-61 Black Widow night-fighter (four) and the Boeing B-29 Superfortress bomber (one in a tail installation, alongside two .50"s) were regular users, although some versions of attack aircraft, and a few American aircraft originally intended for RAF service, were equipped with Hispanos.

An interesting aspect of the P-38 installation was that the gun was mounted in a tightly fitting housing which rigidly connected the front and rear mounting points and to which the ammunition feed was fixed. This produced a self-contained gun/recoil mounting/ammunition feed unit which could be bolted in place without the complexities experienced by the British, at the cost of the extra weight of the housing. This development may have inspired the M3 version which was standardised in July 1944. It was approximately equivalent to the British Mark V, with a shorter barrel, reduced weight and a higher rate of fire, but in addition featured an integral cradle within which the gun recoiled. The Americans also introduced the effective Edgewater mounting (first used on the .50" M2), a friction-type recoil buffer using a series of ring springs arranged to slide partially over each other.

The USN showed rather more interest in the AN-M2 than the USAAF, presumably because they were concerned with preventing enemy bombers from reaching their ships. Following tests of the HS 404 against the .50" Browning, the USN's BuAer Armament Branch concluded that one 20mm was equivalent to three .50"s in destructive power. The AN-M2 was fitted to some of the final piston-engined aircraft such as the F4U-5 Corsair and some F6F night-fighters, but was not regarded as very reliable and saw most use post-war. In general, wartime USN pilots seemed to prefer the HMG, possibly because the combined rate of fire of six .50"s (up to eighty rounds per second) was considered more useful in fighter-versus-fighter combat. The extra destructive power of the AN-M2s was not really needed to deal with the relatively vulnerable Japanese aircraft.

The Americans did spend some effort pre-war in developing their own cannon, and much experimental work was carried out in 23mm calibre, resulting in the .90", which fired a 23 X 139SR cartridge. Four different versions were designed (Tl-4), T2 and T3 being of the API blowback type while T1 and T4 used a long-recoil mechanism. All versions were heavy, slow-firing and unsuccessful, and by 1941 the T4 was being considered as an antitank gun. The 23mm Madsen cannon attracted interest at one time (this fired a different, 23 X 106 round) but this also failed to see service after performing poorly in tests in 1937, as did a 25mm Hotchkiss gun which the French refused to sell

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