The XM556 is a minigun developed by Aeroknox. 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 was designed and engineered around the 5.56mm NATO cartridge. This defensive suppression weapon is significantly smaller and lighter than the M134. It was meant to stand in place of anywhere high covering fire was needed without the weight or footprint of the larger M134 electrically-driven Gatling gun system. The current XM556 Microgun is smaller and lighter than some current 5.56mm belt-fed Squad Automatic Weapons currently on the market, with four times the firepower.
The XM556 is a new platform system that was designed from the ground up. The parts are not just a smaller imitation of the larger M134, but are of a unique design. An absolutely all-new style of bolt was conceived and designed to eliminate current known issues with the M134. The bolts combined with many other improvements have been made to not only extend the life of the gun but reduce wear and reduce or eliminate stoppages as well.
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.