Hailstorm is a minigun and the primary weapon of Mustang.
Hailstorm is a M134 Minigun with a matte green and black body. The minigun is an extremely effective weapon; the weapon is powerful enough to kill ponies and destroy robotic enemies with ease. Mustang acquired Hailstorm whilst exploring the San Palomino Desert, specifically the Moohave or Ponave desert. (He isn't sure which.)
Mustang creates his own ammo for Hailstorm, specifically junk rounds.