View source for Hispano-Suiza HS.820

The '''[[Hispano-Suiza]] HS.820''' was a 20 mm [[autocannon]] developed primarily for aircraft use, but more widely used in a series of ground-based anti-aircraft guns. After [[Oerlikon]] purchased Hispano's armaments division in 1970 the HS.820 became the '''Oerlikon KAD''', supplanting Oerlikon's own [[Oerlikon KAA|KAA]] and KAB weapons in the process<ref>[http://www.janes.com/extracts/extract/jah/jah_0114.html 20 x 139 round (Switzerland)], Jane's Ammunition Handbook</ref>.

Developed in the post-WWII era, the HS.820 fired a 20 × 139 mm round developed from the Swiss 20 x 139 FMK (and FK 38) as a replacement for their earlier 20 x 110 mm design of the widely-used [[Hispano-Suiza HS.404|HS.404]]. The new round was one of two post-war European 20 mm designs, the other being the 20 × 128 developed by Oerlikon for a similar role.<ref>[http://www.quarry.nildram.co.uk/an_introduction_to_collecting_20.htm "An introduction to collecting 20 mm cannon cartridges"], Anthony Williams</ref> The HS.820's high 1100 m/s [[muzzle velocity]] and equally high 1000 rounds per minute firing rate made it a formidable weapon. Nevertheless, the rapid introduction of even higher performance [[revolver cannon]]s meant the HS.820 was never as popular in aircraft role as the HS.404 had been.

The weapon was more widely used in a variety of mounts for the anti-aircraft role, especially in naval use. Hispano introduced a number of mountings with a variety of sighting systems, feeds and power traverse. The '''HS.639-B3''' (later known as the '''GAI-CO1''') was a single-gun mounting fed from a 75-round drum, a fairly serious limitation for a gun with such a high rate of fire. Variants included the '''HS.639-B4''' ('''GAI-CO3''') with a 50-round drum mounted on top of the weapon, and the '''HS.639-B5''' ('''GAI-CO6''') that used dual 75-magazines for longer firing times. The '''HS 665''' was a triple-mount variety using the same drum feeds. The '''HS.666A''' ('''GAI-DO1'''), developed in the mid-70s, featured two HS.820 with new 120-round box magazines mounted on a shared hydraulically powered platform which could be tied into an associated radar-guided fire-control system.<ref>"A Compendium of Armaments and Military Hardware", Christopher Chant, ISBN 0-710-20720-4</ref>

The United States produced a slightly modified version of the HS.820 as the '''M139'''. It saw use on a number of vehicles, including the [[US Army]]'s [[M114 armored fighting vehicle|M114]] and the [[Marine Corps]] [[Amphibious Assault Vehicle]] and the ill-fated [[MICV-65]] series of armored vehicles. The 20 × 139 mm round was also widely used in a variety of other weapons, including the [[Rheinmetall MK 20 Rh 202]] (Mauser Mk.20) and [[20 mm modèle F2 gun]].

==Original Hispano-Suiza Ammunition==
*UIA (HEI) Projectile weight 120 g, Explosive filler 10 g
*RIA (APHEI) Projectile weight 120 g, Explosive filler 4.5 g
*RINT (AP) Projectile weight 111 g, Core weight 70 g

==See also==
*[[Hispano-Suiza HS.404]]

==References==
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[[Category:Aircraft guns]]
[[Category:Automatic cannons]]
[[Category:20 mm artillery]]