From Halopedia, the Halo wiki
The MA5 series, collectively known as the MA5 Individual Combat Weapon System, is a series of air-cooled, gas-operated assault rifles produced by Misriah Armory. Part of the larger MA Series, the MA5 is the workhorse of the United Nations Space Command. Having been in service for more than fifty years, the MA5 series is the oldest rifle platform currently utilized by the UNSC, with more than a dozen models still in use. While the UNSC Marine Corps and the Navy use the MA5 designation, the Army refers to the weapon as the MA37.
The MA5 series is a family of air-cooled, gas-operated, magazine-fed weapons designed for automatic fire. They typically feature an attached electronics suite that provides information on rounds remaining in the magazine and a compass heading, as well as fulfilling several other "housekeeping" functions. These rifles are rugged and sturdy, and perform exceptionally well in a variety of environments, especially given the perceived delicacy of the attached electronics suite. MA5-series weapons generally come configured with a standard-issue flashlight mounted beneath the barrel, but can be fit with an grenade launcher or an underslung shotgun.
The MA5 series rifle is a bullpup rifle and is made of Titanium-A alloy. The rifle is gas-operated and fires from a closed bolt. The charging handle used to chamber the round is situated on the left side of the rifle and moves during operation. Once the first round is fired, the gases from that round and those to follow impinge upon a gas piston, which pushes back the bolt carrier, rotating the bolt inside which unlocks it and extracts the spent brass from the chamber and ejects it. As the bolt carrier group is propelled forward, it chambers another round. This cycle continues until the magazine is empty. Once the magazine is empty, the charging handle must both be pulled back and locked or must be fully cycled after a fresh magazine is inserted to chamber the next round. The magazine itself is located at the butt of the rifle and the magazine release button is located above the magazine well and has an arrow pointing down toward the housing.
The electronics suite is integrated along the top rail guard of the weapon. The MA5 is designed for the toughest of situations, able to be subjected to elements that would otherwise cause a weapon to malfunction, or render a weapon inoperable. The electronics suite contains an LED ammunition counter that displays the number of rounds remaining in the inserted magazine, as well as displaying a magnetic compass. The suite can also project a targeting reticule on the user's heads-up display or neural interface; if such equipment is not available, pop-up iron sights within the cowling can be used.
The MA5 series rifle uses the M118 7.62×51mm Full Metal Jacket, Armor Piercing rounds. These rounds are designed to pierce body armor so the rounds are reasonably large, and are surrounded by a molded jacket of metal, which enhances its armor-piercing capabilities. More exotic ammunition types, like shredder rounds, are available for use, however these modifications are available only by Mission Conditional Requisition (MCR).
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