The prevailing military attitude then envisioned any future conflict as nuclear with defeat or victory decided in hours.

The Rock Island Arsenal converted 1,435 National Match (target grade) M14s by adding a 3-9x Redfield Adjustable Ranging Telescope (ART) and provided National Match grade (7.62 Lake City Long Range XM-118) ammunition.

The ART scope, designed by 2nd Lieutenant James Leatherwood (U.S. Army), combined rangefinding and bullet drop compensation.

The innovation came just in time as the U.S. military found itself losing servicemen to Viet Cong snipers who had the home field advantage in terrain that was, to say the least, difficult.

[12][3] In standard military use, the M21 uses a 20-round box magazine as the other members of the M14 family and weighs 11 pounds (5.27 kg) without the scope.

The M21 was criticized by an internal Fort Benning publication in 1989 warning that "The M21...cannot be maintained under field conditions, and its inflexible design makes it highly susceptible to malfunctions" due to the scope not being easily removable if needed and the glass bedded action that made disassembly in the field problematic.