The XM556 is a rotary machine gun developed by Empty Shell LLC. The weapon is chambered in 5.56×45mm NATO. The "XM" prefix is for marketing purposes: the weapon is not currently being tested by any military.
The XM556 Microgun is intended as a high volume-of-fire support weapon, effectively a portable M134.
Unlike the XM214 Microgun, the XM556 is not just a scaled-down M134. A new style of bolt was designed for it, claimed to eliminate "known issues" with the M134. Many other alterations have been made aimed at increasing lifespan and reliability.
The XM556 is 5.56mm caliber, externally-powered Gatling-type machine gun capable of an extremely high cyclic rate, of 4000 to 8000 rounds per minute.