M655 machine gun

The M655 Heavy Machine Gun/Anti-Matériel is a 20mm heavy machine gun used by the UNSC Defense Force. Some versions of the Mark IX "Mantis" powered exoskeleton are armed with an M655 as well as an M5920 surface-to-surface missile launcher.

Trivia
Under modern military conventions, the M655's 20mm caliber would actually classify it as an autocannon.

List of appearances

 * Halo 4