The M21 Sniper Weapon Systemis chambered for the 7.62x51mm NATO cartridge. Sniper AMMO

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) M14s by adding a Leatherwood 3–9x Adjustable Ranging Telescope and providing National Match grade ammunition. It was designated the M21 in 1975. The M21 remained the Army's primary sniper rifle until 1988, when it was replaced by the M24 Sniper Weapon System.

In standard military use, the M21 uses a 20-round box magazine as the other members of the M14 family, and weighs 5.27 kg without the scope. The U.S. military never officially authorized or purchased magazines in any other capacity, although 5- and 10-round magazines are available.