The US Rife Caliber 7.62mm M21 is the sniper variant of the M14 Rifle. The M21 was originaly fielded in Vietnam by the US. It was developed in he mid to late 1960s by the US Army Marksmanship Training Unitat Ft. Benning, Georgia as an offshoot of the M14 National Match Rifle program, where a detailed procedure was developed for taking the service grade M14 and converting it into a highly precice match rifle for competitive target rifle shooting. In both the M14 National Match and the M21, the rifle was fitted with a more accurate, non chrome-lined barrel stamped "NM", a precisley fitted and welded gas system, more precise sights and the rifle, stock and handguard are carefuly "bedded" together using a epoxy compound. The stock itself was inpregnated with polyurethane in a special pressure cooker to make it less suceptable to warping with changes in ambient humidity. In addition, the M21 was fitted with a Leatherwood Adjustable Ranging Telescope (ART) and mounting system. This unique scope encorporated a reticle that enabled the shooter to quickly range the intended target and, while doing so, automaticaly adjust the scope to permit a first-round hit from the rifle at any practical range. The rifle was used in Vietnam with great success both during the day when using the ART scope or at night with a starlight scope and, in some cases, a supressor. However, this weapon system suffered from excessive downtime in armorers workshops. The fragile National Match bedding, the ART scope, and other modifications and additions often failed under the rigors of combat and Army Ordnance personel in theater had a difficult time keeping an adequate number of rifles in service. These problems were later partialy corrected in the very similar Army M25 and USMC Desiganted Marskmans Rifle programs in the '80s and '90s, with improved materials and procedures resulting in more robust (but still troublesome) weapon systems. The M21 itself still soldiers on in limited service with the US Army in the hands of non-sniper troops that require a precision rifle, such as the Military Police.