The M21 was developed at the request of the army during the Vietnam war when the need for an accurate, long range sniper rifle arose. The gun was based off the M14 weapon platform due to it's accuracy, reliability, and ability to fire off a quick second shot, a quality it retained in COD4 multiplayer. The gun was converted from the M14 by the Rock Island Arsenal by adding a Leatherwood 3-9x adjustable scope and by providing a National Match grade barrel. It was called the XM21 until 1975, but had been unofficially called the M21 since 1969. The M21 was the Army's official sniper rifle until 1988, when it was replaced by the bolt-action M24, although it has still seen extensive use in Iraq and Afghanistan.