During the military and political chaos occuring in AC 195, pilot Trowa Barton was forced to abandon his XXXG-01H Gundam Heavyarms mobile suit, leaving it behind on Earth when he went back out into space. Over the next few months, the abandoned Heavyarms would find its way into the care of guerilla rebel Sally Po. Sally, realizing the chaos would be concluded in space and that the infamous Gundam pilots would be at the very center of it, would later go into space herself, taking the Gundam with her to return it to its pilot. Aboard the independent battleship Peacemillion, Gundam supporter Howard would upgrade the groundpounding Heavyarms, outfitting it to fight in space.

The improvements made to the Gundam Heavyarms Kai were necessary to give the mobile suit the edge it needed to deliver its firepower against the spacebound forces of White Fang's Virgo II mobile dolls and the mobile suits of the World Nation military. A minor improvement was the addition of several vernier thrusters to give the Heavyarms stability in a zero-G environment. Its body houses twice as many missiles, and its chest now contains four gatling cannons. The biggest improvement was the replacement of its single, large beam Gatling gun with a two pairs of double-barreled shell-firing Gatling guns, allowing the Heavyarms to split its devastating firepower up and tackle more targets at once. However, its right forearm no longer sports a heat knife, the only melee weapon of the original Heavyarms.

Source: Gundamproject.com, Gundam W: Endless Waltz DVD