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 KAA and KAB weapons in the process.
Developed in the post-WWII era, the HS.820 fired a 20×139 mm round developed from the Swiss 20×139 FMK (and FK 38) as a replacement for their earlier 20×110 mm design of the widely-used 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. The HS.820's high 1100 m/s muzzle velocity and equally high 1000 round per minute firing rate made it a formidable weapon. Nevertheless, the rapid introduction of even higher performance revolver cannons 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-round 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.
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 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
- 20 x 139 round (Switzerland), Jane's Ammunition Handbook
- "An introduction to collecting 20 mm cannon cartridges", Anthony Williams
- A Compendium of Armaments and Military Hardware, Christopher Chant, ISBN 0-7102-0720-4