The HS.820 is a 20 mm caliber autocannon developed by Hispano-Suiza primarily for aircraft use, but more widely used in a series of ground-based anti-aircraft guns.

Nevertheless, the rapid introduction of even higher performance revolver cannons meant the HS.820 was never as popular in an aircraft role as the HS.404 had been.

Hispano introduced a number of mountings with a variety of sighting systems, feeds and power traverse.

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 uses dual 75-round magazines for longer firing times.

The HS.666A (GAI-DO1), developed in the mid-1970s, features 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.