The M21 sniper rifle is a gas operated, semi-automatic rifle which uses Garand-type rotary bolt locking and gas-operated short stroke piston system, located below the barrel. The M21 rifle retained adjustable iron sights of M14 rifle, and added a telescope or night sight using standard M14 rifle side mount (each military M14 rifle was manufactured with provisions for mounting a scope bracket on the left side of the receiver). For special operations, the M21 rifle was often issued along with a Sionics sound suppressor (silencer).

Starting out as the XM21, this sniper rifle was developed jointly by the Army Weapons Command at Rock Island, Combat Development Command at Ft. Benning, and the Limited Warfare Agency at Aberdeen, during late 1960s. The XM21 was an accurized M14 National Match (NM) semi-automatic rifle equipped with a Leatherwood 3X-9X Adjustable Ranging Telescope (ART). The Rock Island Arsenal converted 1,435 M14NM rifles to XM21 sniper rifles for initial fielding to Vietnam in 1969. The rifle was initially fielded with a wooden stock, which was later replaced with a fiberglass stock. The XM21 was officially type classified M21 in 1975, and remained a standard US Army sniper rifle until 1988, when it was officially replaced with M24 SWS. The M21 was accurate to 750 yards (690m). The rifle used 7,62mm NATO M118 ammunition, especially developed for sniping and long range match shooting. The ART telescope featured a variable magnification power of from 3X to 9X, for adjustable ranging between 300m and 900m. This adjustable ranging feature removed much of the guesswork from aiming at the target. The ART was ballisticly matched with M118 ammunition. Later on, a modified M25 sniper rifle was developed for US Scecial Operations forces. It retained basic M14 / M21 mechanisms but added a McMillan polymer stock, some commercially available parts (such as replacement gas piston), and a new scope mount. M25 rifles are ususally issued with more modern Baush & Lomb or Leupold telescope sights. Older Sionics suppressor is replaced by Ops Inc suppressor. Finally it must be noted that more than few M21 rifles were recently re-issued to US forces in Iraq and Afghanistan to complement older M24 rifles before enough M110 semi-automatic rifles will be available. Some old M14 rifles also were recently converted to M21 configuration as well. Most such conversion and re-issues use new generation scope mounts with Picatinny rails and new-generation telescope sights.

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