US Cartridge, 20mm HEI, M56A1, M56A2, M56A3, M56A4
These are electrically primed cartridges with high-explosive-incendiary projectiles and centrifugally armed, point-detonating fuzes. They are used in the M39, M61, M168, and M195 automatic cannons.
The cartridge case is unpainted but has nomenclature and loading information stenciled on it. Cartridges of current manufacture have projectiles painted yellow with a red band below the fuze. Projectiles of earlier manufacture were painted yellow overall, or red with an olive drab ogive. Nomenclature and loading information is stenciled in black on the projectile body and may be stamped in the rotating band. The fuze is unpainted and unmarked.
The models differ in the method of loading the projectiles. The M56A3 has the HE mix and the incendiary mix combined in one pellet. The M56A4 has the incendiary pellet inserted into the projectile, then the HE mix pellet is added. This improves fire start capability.
Collaborative Ordnance Data Repository