From Halopedia, the Halo wiki
The MA series is a series of air-cooled, gas-operated assault rifles produced by Misriah Armory that are used by the United Nations Space Command Armed Forces. They are chambered for a variety of cartridges, though the most common is the 7.62×51mm round, with specialist ammunition available via request. They can be fired at a full range settings, though the most common is fully automatic. All iterations of the MA series are presumably bullpup weapons, with the receiver and feeding system located behind the trigger. The weapons of the MA series are rather heavy, though they are easy to clean and use. They are capable of performing in a variety of combat scenarios. The MA series includes the MA5 subseries, which has served as the UNSC's primary service rifle since 2437. The weapons of the MA5 series, and presumably the whole of the MA series, are capable of being outfitted with M301 Grenade Launchers and underslung shotguns.
List of appearances