The M21 was a popular UNSC and Allegiance sidearm during the Swarm War. Essentially a continuation of the M6 Series of pistols from the Human-Covenant War, it utilised a large calibre pistol round, together with several recoil-reducing mechanisms and sturdy construction to produce a well-rounded weapon. It had good range, accuracy, reliability and, most of all, dealt large amounts of damage. All of these factors made it more favoured than most weapons by UNSC servicemen and women, making it the de facto service pistol throughout the Swarm War. It fired potent 12.7x40mm rounds, giving the weapon great stopping power and compensating for its relatively low rate of fire.
Heavily inspired by older, equally powerful defence weapons such as the M6G, the M21 attempted to retain these characteristics while at the same time improving usability and controllability, which naturally suffered somewhat from the weapon's recoil. While not completely meeting these criteria, it was sufficiently wieldable to be used as a fallback personal defence weapon, albeit mainly in the hands of experienced users. Although many other compact submachine guns and pistols offered better control and accuracy, the M21 had exceptionally high stopping power, far more so than its counterparts. While this initially lent it dislike from many service personnel, it came into its own during the Swarm War, where its high power and accuracy enabled it greater potency against the Swarm's exoskeleton. As a result, it was famed for being a dependable and powerful weapon throughout the war. The weapon was designed by the UNSC but officially adopted by the Allegiance in 2710.
The M21 was used almost exclusively as a personal defence weapon, as a fallback in case the user's primary weapon was damaged, lost or empty of ammunition, or otherwise unusable. As it had a longer range than most other pistols, it was often used in more open and expansive battlefields, or when more than the usual stopping power was needed. It was quite large by pistol standards and rather heavy too, which reduced but did not negate the substantial recoil. Used as the sidearm of many personnel operating behind friendly lines, which did not detract from its combat abilities and only added to its popularity. It was widely known as the best ballistic sidearm for piercing armour; while the round was developed for piercing personal body armour, the Swarm used a natural exoskeleton similar in composition and design to artificial armour. While less skilled soldiers found the weapon unusable and useless, more experienced troops could make the most of the weapon and employ its potency to the best of the weapon's abilities. Even still, it was often carried alongside more conventional personal defence weapons as a way of ensuring maximum protection.