Mark 2488 Onager

The 15cm Mark/2488 1.1GJ MAC, also known as the Onager or often just referred to as a "Mass Driver", was a Magnetic Accelerator Cannon used by the United Nations Space Command during the Human-Covenant War.

Design details
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 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.

Operational history
During the Battle 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 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.

Trivia

 * 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 medieval siege weapon, which in turn 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 the line delivered by Master Chief Petty Officer John-117 in the opening cinematic of the level Cairo Station in Halo 2.
 * The Onager is strikingly similar in appearance and function to the ship-mounted railgun used by the United States Navy in Transformers: Revenge of the Fallen.
 * The Onager has "S-239" painted on it's hull. The service tag of Emile.

List of appearances

 * Halo: Reach