Mark 2488 Magnetic Accelerator Cannon

The 15cm 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.

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.

Gameplay
The Onager is only operated once in Halo: Reach, on the last level The Pillar of Autumn. It is used to destroy passing Phantoms and allow Noble Six to halt the attack of a CCS-class Battlecruiser.

Trivia

 * The Bungie.net stats description for the Onager is simply "I need a weapon", a reference to the line delivered by John-117 in the opening cinematic of the level Cairo Station in Halo 2.
 * The Onager is named after a medieval siege weapon, which takes its name from a horse subspecies.
 * It is possible that the Onager was brought in with the Pillar of Autumn and was assembled, on a rooftop lookout near the platform that Keyes' Pelican lands at, so troops could assist the Halcyon-class Cruiser at defending it. Also, it wouldn't have been a stationary MAC Gun because the base of the MAC has what seems to be automated bolts on the outer rim of the base.
 * The Onager is striking similar in fashion with the prototype Railgun in Transformers: Revenge of the Fallen. Both fire a beam-like projectile and are strikingly similar in appearance.

List of appearances

 * Halo: Reach