The HPAO-series of weapons were heavy-caliber weapon systems designed shortly after the inconclusive/UEG victory over Vespene for the Unity's need to combat the human tactic of landing marine troops from orbit from their warships without bombarding the planet beforehand. The cannon was soon placed on Vespene just several weeks after the first battle there. When the UEG invaded a second time, the HPAO-01 worked successfully, destroying several frigates in just several shots and damaging heavier destroyers and cruisers. Unlike the -03 and -04 models (similar to the -01 and -02 models respectively), the -01 and -02 models fired solid-steel slugs (similar to that used by humanity), a powerful 510-mm shell able to tear apart a Sparrow-class landing shuttle in a single hit.