The United States Army wanted an accurate sniper rifle during the Vietnam War. It selected the M14 because of its accuracy, reliability, and the ability for a quick second shot. Adapting it to the M21 sniper rifle was accomplished by adding an Adjustable Ranging Telescopic (ART) scope.
The M21 uses 10 round box magazines, and weighs 5.27 kg without the scope. It is still popular today, being the main long sniper rifle used by the U.S. Army.