A few historical details: The 20mm cannon used by the RAF, USAAF and USN started life as the French Hispano-Suiza HS 404. It was very powerful for a 20mm, firing a 130g projectile at around 880 m/s, and it fired at a fast 700 rpm. Main problem was it was designed for engine mounting (as in the Dewoitine D.520) and was very long and heavy. It also only had a 60-round drum magazine.
The British adopted it as the Hispano, more or less as was, but slowed the RoF to 600 rpm in the interests of reliability. It entered service in late 1940, but by 1941 a belt feed had been devised and this rapidly replaced the drum feeds.
The US adopted it as the AN-M2 (AN signifying it was for both the army and navy). However, for various complicated reasons the US had all sorts of production difficulties and it was never very reliable.
Towards the end of the war, the RAF and US both introduced improved versions, which were shorter, lighter (around 42 kg instead of 50 kg), and faster-firing (750 rpm). These were the Hispano Mk V and M3 respectively. They lost a bit of muzzle velocity and hitting power due to the shorter barrels (around 850 m/s) but were among the best 20mm cannon of the war. However, the US production problems were not really resolved until after the war. This might account for the continued popularity of the .50, even though the USN rated the 20mm as three times as effective.
Tony Williams
Author: Rapid Fire - The Development of Automatic Cannon, Heavy Machine Guns and their Ammunition for Armies, Navies and Air Forces.
Details on my military gun and ammunition website:
www.users.globalnet.co.uk/~autogun/ [This message has been edited by Tony Williams (edited 02-23-2001).]
