The M21 Sniper Weapon System (SWS) is the semi-automatic sniper rifle adaptation of the M14 rifle. It is chambered for the 7.62×51mm NATO 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 follow up shot. As a result, in 1969, the Rock Island Arsenal converted 1,435 National Match (target grade) M14s by adding a Redfield ART Adjustable Ranging Telescope 3–9×.