The Mark/2488 1.1GJ MAC, also known as the Onager or "mass driver," was a Magnetic Accelerator Cannon used by the United Nations Space Command during the Human-Covenant war. An upgraded version, the Mark 2551 Portable Magnetic Accelerator Cannon, is in use after the war.
The Onager is capable of firing a 15 cm round with up to 1.1 gigajoules of kinetic energy. It has five linearly arranged magnetic coils, each of which needs to charge individually before each powerful shot. Additionally, the weapon needs to cool off after every shot, and does so by opening a radial vent on the upper portion of the barrel. The weapon is operated from a small booth on the left side of the cannon.
During the Fall of Reach, a group of UNSC Marines installed the Onager cannon on top of one of Reach's industrial facilities in Aszod to protect the UNSC Pillar of Autumn while drydocked. Emile-A239 manned the mass driver to fend off incoming Covenant Banshees and Phantoms before the Autumn could take off. However, the SPARTAN-III was killed by two Sangheili warriors, prompting SPARTAN-B312 to commandeer the cannon and successfully delay the Covenant assault on the Autumn by crippling an incoming CCS-class battlecruiser with a shot to the ventral energy projector.
- This weapon is only seen once throughout the course of Halo: Reach, featured on the level The Pillar of Autumn.
- It's named after a Ancient Roman siege weapon, which takes its name from a horse subspecies.
- For the Bungie.net weapons statistics, the description for the Onager is simply "I need a weapon," a reference to John-117's line in the opening cinematic of the level Cairo Station in Halo 2.
- The Onager has the characters "S-239" painted on it, which is the service tag of Emile-A239.
- The Onager is the most powerful usable weapon in the games, as it is able to destroy Phantoms in one shot and is able to damage a Covenant capital ship.
- Halo: Reach (First appearance)