The M21 Sniper Weapon System (SWS) is the semiautomatic sniper rifle adaptation of the popular M14 rifle. It is chambered for the 7.62x51mm cartridge.
The United States Army wanted an accurate sniper rifle during the Vietnam War. The M14 was selected because of its accuracy, reliability, and the ability for a quick second shot. As a result, in 1969, the Rock Island Arsenal converted 1,435 National Match (target grade) M14 rifles by adding a Leatherwood 3–9x Adjustable Ranging Telescope (ART) telescopic sight and furnishing National Match grade ammunition. It was designated the XM21 until 1975, when it became the M21 (although it had been unofficially called the M21 since December 1969).
The M21 remained the Army's primary sniper rifle until 1988, when it was replaced by the bolt-action M24 Sniper Weapon System, which itself is currently in the process of being replaced with the M110 Semi-Automatic Sniper System (SASS). While bolt action rifles are generally more accurate at long distances (1000 m) than semiautomatic weapons, semiautomatic rifles are more than adequate for medium distances (600 m). Thousands of stored M21s are being modernized with new scopes and stocks to be re-fielded among American troops, as well as M14s being converted to sniper rifle roles, which will be used until the M110 gets widely fielded and possibly alongside it.
In Game
In single-player the M21 is the starting secondary weapon in Blackout, and the starting primary weapon in All Ghillied Up and One Shot, One Kill.
The M21 can be unlocked in Call of Duty 4 multiplayer at level 7. It is the second sniper rifle obtainable (the first being the M40.) The M21 is useful in many situations,as it has very low recoil and is. What it lacks in its damage, its higher capacity magazine and increased rate of fire make up for. It is the only sniper rifle that cannot kill in one shot with stopping power perk enabled and because it is semi automatic it is highly inaccurate when not holding breath.