From Halopedia, the Halo wiki
The M90 Close Assault Weapon System (CAWS), also known as the M90 Close Assault Shotgun, M90 assault shotgun, or simply the M90, is a pump-action shotgun in service with UNSC. It is a military variant of the prolific DTM series of shotguns from Weapon Systems Technologies.
The M90 is a pump-action shotgun, chambered in the extremely powerful 8-gauge magnum shell, and feeding from a 12-round, dual-tubular, non-detachable magazine. It is generally utilized alongside MAG 15P-00B rounds, and is designed as a more practical and cost-effective recreation of Misriah Armory’s M45E Tactical Shotgun, though in UNSC service it is considered to be functionally identical.
The magazine of the weapon is prominently positioned above the barrel, with the weapon’s glowing iron sights mounted atop it and the receiver located just behind, facing upwards, roughly in-line with the trigger. The magazine possesses a dull grey or glossy black finish with ribbed indents along its length. A flashlight is integrated into the weapon’s forend, which also features a prominent reddish button.
The M90 shotgun is hefted like a rifle, with the user's dominant hand holding the primary pistol grip, and their off-hand supporting the weapon by holding onto the forend (the portion of the weapon which is acted upon by the user to facilitate the pump action, which also acts as a forward grip for the off-hand). As is typical of human firearms, in order to discharge the weapon, the trigger positioned at the top of the pistol grip must be depressed. If the weapon is loaded with standard ammunition, the weapon's firing will take the form of a spray of pellets, referred to as "buckshot", which are quite powerful at close range but lose damage efficiency as the spray disperses and loses energy. The firing of the weapon produces a fiery muzzle flash as burning propellant gasses leave the barrel.
Once the chambered round is fired, to repeat, the forend of the weapon is slid back across the weapon’s underside in the direction of the stock, retracting the bolt and removing the spent shell from the firing chamber. When the forend is fully retracted, the spent shell will have been ejected, and the way will have been cleared for the elevator component to position a new round for the bolt to carry into the chamber when the slide is once again pushed forward in the direction of the muzzle. Once the weapon’s magazine is expended, new rounds must be inserted one at a time into the receiver to continue firing. It is possible at any time to top up the magazine with additional ammunition until it is full.
The M90 is a devastating weapon in close, confined environments, with a loud report and strong recoil. The spray of pellets from the weapon mean that at slightly longer ranges, it can be used to hit clusters of targets simultaneously, but also makes the weapon inefficient at extended ranges due to the low ratio of shot which hits the target to the amount of shot fired. The M90's powerful cartridge makes it capable of efficiently engaging and defeating enemies with energy shielding within effective range. The Covenant learned to be wary of this weapon in urban environments, noting its close-range lethality.
The M90 was a common close-range battle implement in the service of the UNSC throughout the Human-Covenant War. It saw service in the hands of John-117 during Operation: SILENT STORM, but its most prominent usage came in 2552. It played a notable part in the Battle of Installation 04, being used by UNSC Marines against the Covenant and Flood. An M90 was also present during the ensuing Skirmish over Threshold. It went on to be widely utilized during the Battle of Earth. Notably, the Master Gunnery Sergeant who oversaw John-117's issuing with his new MJOLNIR Mark VI armor was killed on Cairo Station while using an M90 to defend his armory from a pair of Sangheili. M90s were brought on the UNSC In Amber Clad, and thus were employed throughout the Battle of Installation 05. During the battle, many Marines wielded M90s during their incursion into the quarantine zone. Following the accidental release of the Flood by Thel 'Vadamee, many of these Marines fell victim to the parasite, with their M90s remaining in the possession of the resultant combat forms. Towards the end of the Battle of Installation 05, during the opening stages of the Great Schism, many M90s were captured by Jiralhanae forces, who employed them against both separatist Sangheili on the ring's surface, and Flood during their invasion of High Charity.
The original Halo: Combat Evolved depiction is different in three notable aspects from the later depictions in Halo 2 and Halo 2: Anniversary. The first is that the weapon features a smooth magazine lacking a heatshield, with a distinctive chrome finish. The second is that the luminous iron sights glow blue instead of green. The final differentiation is that in Halo: Combat Evolved, but not successive depictions, when the weapon is reloaded after being completely emptied, the weapon’s action is worked to chamber the first round, which is considerably more realistic. A similar shotgun design, the M90A shotgun, is featured in Halo 3 and Halo 3: ODST.
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