Mark 2551 Onager
The Mark 2551/25cm Portable Magnetic Accelerator Cannon, also known as the Onager, is an upgraded variant of the Mark 2488 Magnetic Accelerator Cannon developed by Misriah Armory. It uses a 25cm ferric-tungsten depleted uranium round that is fired at hypersonic speeds.
The UNSC's postwar prototype flagship, UNSC Infinity, is equipped with a number of Mark 2551 MAC dorsal turrets. During the initial battle on the Forerunner shield world Requiem in 2557, the "MAC network" controlling these turrets was disabled by troops of Jul 'Mdama's Covenant fighting for the Didact, through the use of three jamming devices placed on the Infinity's hull. Once these jammers were destroyed by John-117, Cortana successfully brought the network back online in a few minutes, though a link relating to "EM relays" needed to be manually initiated before they could fire. When they did, they successfuly forced the Didact in his Cryptum and his allies to retreat. Along with Infinity's anti-aircraft batteries, the turrets shot down at least two CRS-class light cruisers.
ONI's Ivanoff Station near Installation 03 made use of several automated Mk. 2551 Onagers for defense against space debris and later against at least three Covenant CRS-class light cruisers when the station was attacked by Covenant forces under the command of the Didact himself. The station's defenses were reprogramed by Cortana to give the evacuating personnel covering fire against the attacking Covenant forces. The Onagers shot down multiple approaching Phantoms, though some managed to slip through their defensive fire to land troops on the station.
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