From Halopedia, the Halo wiki
The M6 Personal Defense Weapon System is a series of pistols produced by Misriah Armory. Common slang terms for the M6 series handguns include go-mag, hand cannon, power drill, HE pistol, and heavy pistol.
The M6 series have seen major widespread distribution through the UNSC Armed Forces as lighter firearms, although they are not always seen as primary weapons, because of their lighter penetrating power. They have seen usage in the UNSC since 2414. Starting in 2555, the M6H has taken prominence over other variants and has been designated the standard side arm of all UNSC branches.
The M6 series weapons are recoil-operated and magazine-fed, and typically fire 12.7×40mm projectile ammunition, and despite the large caliber, oftentimes "exotic" ammunition is not utilized. They are also remarkably large and overpowered to an extent, appearing to be redolent of a cross between a pistol and a rifle in size. In addition to power, the M6 line of weaponry is designed to be exceedingly rugged and reliable, and the guns are able to remain operational after Slipspace transitions and high-g aerial maneuvers, as well as the rigors sustained during standard combat. However, the heavy weight limits its distribution amongst personnel, and the M6 weapons have a tendency to drop casings on the wielder's foot, leading some UNSC personnel to favor the M7 SMG, which operates on a caseless system.
They have remained remarkably unchanged during their decades of service, although there are a large number of variants. The M6 series handguns are issued in three different finishes: hard chrome, black polymer, and electroless nickel. Some variants of the M6 series have been 'up-sized' to approximately 117% their normal size to better facilitate use by Spartans.
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