From Halopedia, the Halo wiki
The 8-Gauge Magnum (3.75") shell is a size of shotgun cartridge used in the shotguns of Misriah Armory's M45 Tactical Shotguns and Weapon System Technologies military-variant DTM series shotguns. This ammo type is also known as the Soellkraft Hippo 8-gauge magnum shell, relating to the cartoon hippo face decorating their exterior.
The 8-gauge cartridge was a type of ammunition most commonly employed in 19th century firearms with antiquated black powder propellant, considered by the 21st century to be oversized and overpowered for shotgun applications. Nevertheless, the round has re-entered common usage with military and security forces in the 26th century and serves a role as a munition for extremely powerful assault weapons. The shell is employed with both 0000 and 00 projectiles, both of which are buckshot ammunition, meaning that they fire a spray of small pellets or "shot" as opposed to a single bullet. In length, the cartridge is 3.75". Due to its sheer firepower, it is among the relatively few UNSC ammunition types that can be efficiently and consistently used to engage targets with personal energy shielding.
8-Gauge Magnum shells filled with 0000 ("quadruple-ought") shot are the standard ammunition type for the UNSC's M90A and M45 series shotguns. Each of these projectiles leaves the barrel of an M90A shotgun travelling at 300-400 meters per second, and a shell of this type fired from an M90A has an effective range of 40 meters. Shells of this type used with the M45D shotgun have been known the bear the designation "M296 SC" "M296 TS", and "M296 Magnum".[Note 1]
MAG 15P-00B (Magnum, 15 Pellets-00 Buck) are 8-gauge magnum shells filled with 00B ("double-ought buck") shot, and are frequently utilized with the M90 shotgun. According to the designation, each shell contains 15 pellets. It is considered extremely powerful in confined, close-range engagements, but loses effectiveness at longer ranges due to the dispersal of the shot. At medium range, however, the spread of pellets can be used to engage more than one target simultaneously.
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