The Gundam Ez8 Heavy Armed Custom (Gundam Ez8 HAC for short) is a custom variant of RX-79(G)Ez-8 Gundam Ez8 piloted by Shiro Amada. It first appeared in the SD Gundam G Generation Gather Beat video game. It's existence contradicts the continuity of Mobile Suit Gundam: The 08th MS Team, and is thus considered to be non-canon.
Gundam Ez8 HAC is actually one of two optional equipment packs for RX-79Ez-8 Gundam Ez-8 Kai which is Gundam Ez-8 modified for space combat. The HAC equipments is focus on firepower, its main armaments are dual large beam cannons modified from Salamis-class cruiser's mega particle cannon. Gundam Ez-8 HAC also has pair of manipulator arm on its legs to hold the Salamis cannons in firing position.
However, the modification to basic frame of Gundam Ez-8 remain minimum. Gundam Ez8 HAC has quite low mobility due to the increase weight of HAC equipments.
Although the Gundam Ez8 Kai and both variants aren't fit to the canonical timeline, Gundam Ace magazine made a claim in Game MSV column that the Gundam Ez8 Kai is actually fictional mobile suit appear in drama program based on experience of the 08th MS Team members. Since the drama was exremely popular, the story got extend and eventually has the 08th MS Team going to space.