Mark 2488 Onager
From Halopedia, the Halo wiki
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.
The Mark 2551/25cm Portable is an upgraded variant of the weapon brought into service 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 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.
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 Asźod to protect the drydocked UNSC Pillar of Autumn. 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.
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