[hattrick]

The United States Marines 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.

wiki - m21 rifle

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