Mark 2488 Onager

The Mark 2488 Magnetic Accelerator Cannon, formally designated 15cm Mark/2488 1.1GJ MAC, also known as the Onager and often referred to as a "mass driver", is a stationary 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. 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.

Mark 2551
The Mark 2551/25cm Portable uses a 25cm M6792 HV/FTHPP-AS round and developed by Misriah Armory. The had a hull-mounted Onager in support of its larger shipboard MACs. ONI's Ivanoff Station near Installation 03 made use of several automated Mk. 2551 Onagers to protect it from space debris and later against several Covenant CRS-class light cruisers when the station was attacked.

Operational history
During the Fall of Reach, a group of UNSC Marines installed an Onager on top of one of the planet's ship-breaking facilities in Aszod to protect the drydocked. Emile-A239 manned the mass driver to fend off incoming Covenant Banshees and Phantoms before the Autumn could take off. However, the SPARTAN was killed by a Sangheili Zealot, 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 its ventral energy projector.

The UNSC's postwar prototype flagship,, is equipped with a number of dorsal turrets quite similar to the Mark 2488. During the initial skirmishes on the Forerunner shield world Requiem in 2557, these turrets were used to fend off the Didact's Cryptum, forcing the Didact to retreat. Several automated Onagers were mounted across Ivanoff Station and were used by the UNSC forces to defend the Composer from invading Covenant remnant forces under the command of the Didact himself.

Trivia

 * The Onager named after a medieval siege weapon, which in turn takes its name from a horse species.
 * On the Bungie.net weapons statistics page, the description for the Onager is simply "I need a weapon", a reference to a line by John-117 in the opening cinematic of the level Cairo Station in Halo 2.
 * "S-239" is painted on the Onager's frame. This is a reference to Emile-A239, who was killed while operating the weapon.
 * The "using Onager early" glitch can be exploited so that a player can reach the driver before the scripted cutscene. This glitch can provide a player with an easy killing spree by shooting down Banshees, or avoiding enemy lances by shooting the Phantoms carrying them.

List of appearances

 * Halo: Reach
 * Halo 4