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The XM21 sniper rifle was developed jointly by theArmy Weapons Command at Rock Island, Combat DevelopmentCommand 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 AdjustableRanging Telescope (ART). The Rock Island Arsenal converted 1,435 M14NMrifles to XM21 sniper rifles for initial fielding to Vietnam in 1969.The rifle was initially fielded with a wooden stock, which was laterreplaced with a fiberglass stock. The XM21 was officially typeclassified M21 in 1975, and remained a standard US Army sniper rifleuntil 1988, when it was officially replaced with M24 SWS. The M21 wasaccurate to 750 yards (690m). The rifle used 7,62mm NATO M118ammunition, especially developed for sniping and long range matchshooting. The ART telescope featured a variable magnification power offrom 3X to 9X, for adjustable ranging between 300m and 900m. Thisadjustable ranging feature removed much of the guesswork from aiming atthe target. The ART was ballisticly matched withM118 ammunition.
Later on, a modified M25 sniperrifle was developed for US Scecial Operations forces. It retained basic M14 / M21 mechanisms but added a McMillan polymer stock, somecommercially available parts (such as replacement gas piston), and anew scope mount. M25 rifles are ususally issued with more modern Baush& Lomb or Leupold telescope sights. Older Sionics suppressor isreplaced by Ops Inc suppressor.
Finally it must be noted that more than few M21 rifles were recently re-issuedto 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 suchconversion and re-issues use new generation scope mounts with Picatinny railsand new-generation telescope sights.
M21 sniper rifle isgas operated, semi-automatic rifle which uses Garand-type rotary boltlocking and gas-operated short stroke piston system, located below thebarrel. M21 rifle retained adjustable iron sights of M14 rifle, andadded a telescope or night sight using standard M14 rifle side mount(each military M14 rifle was manufactured with provisions for mountinga scope bracket on the left side of the receiver). For specialoperations, M21 rifle was often issued along with Sionics soundsuppressor (silencer).