M21

The M21 Sniper Weapons system is the semi-automatic sniper adaptation of the M14.

Overview
In 1969, it was developed and entered the United States armed service as a sniper rifle. It was used frequently in the Vietnam War, although it has been to little conflict outside of Vietnam.

During the Peace Walker Incident in 1974, Big Boss and his mercenary group, the Militaires Sans Frontieres, procured these design specs shortly after capturing Zadornov for the fourth time. It was modified by MSF's R&D Team by trimming down the barrel, and later by adding a suppressor.

Post-Peace Walker
It was retired from the service in 1989.

In-game weapons description
"A customized sniper rifle version of the M14, predecessor of the M16 as the U.S. Military's standard assault rifle. The M21 takes the M14, already known for its superb accuracy, and makes it even better through accuracy-boosting sniper rifle modifications. In addition to its high precision, the M21 is equipped with a 3-level zoom scope to assist in targeting at various distances. Moreover, it fires armor-piercing rounds that can take even a well-protected foe down from a distance with a single headshot straight through the helmet. (Rank 4) This model features a shorter barrel, which results in slightly higher recoil but makes it much easier to handle and carry, qualities that make it a solid weapon for scouts. (Rank 5) Equipped with a limited-use suppressor. The suppressor will eventually wear out, so use it prudently."

- M21 weapons descriptions from Metal Gear Solid: Peace Walker