|Damage Per Hit||
15 cm ferric-tungsten slug
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.
At least one such Onager has been installed at the top deck of the UNSC Infinity and was used remotely. This Onager aided in UNSC Infinity's take off when it crashed landed on Requiem, and also assisting in the retreat of the Didact. A number of Onagers were also used for the defense of Ivanoff Research Station.
- 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 in turn takes its name from a horse subspecies.
- The Roman Onager was named for the violent kick of the throwing arm, which is an apt name for this artillery piece, considering the amount of recoil that would be generated by a Magnetic Accelerator Cannon
- 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 the characters "S-239" painted on it, which is the service tag of Emile (A-239).
- The text when entering the Onager erroneously calls it a "MAC Cannon", despite MAC standing for Magnetic Accelerator Cannon, meaning the word "cannon" is repeated twice.
- The Onager is quite possibly the most powerful vehicle/turret that can be used in the games as it is able to destroy Phantoms in one shot and is able to damage a Covenant capital ship.
- The weapon is so powerful that when it hits its target, the entire map turns gray for a split second. (This can easily be viewed in Theater mode)
- The Onager is mobile, since it is shown to have a similar base design like the stationary Gauss Cannon and LAAG Turret. It is possible that the UNSC Pillar of Autumn was carrying/transporting this turret and ordered UNSC troops to deploy the turret to defend it.