As is common knowledge, the military forces in the 20th century had a very bad habit of disbanding sniper units and training between the major conflicts and after WWII it was no different for the US Army. So while they had gone through the effort and cost during WWII to first field the stop gap M1903A4 sniper rifle, and then develop and field the M1C and M1D sniper rifles, once the war ended, those rifles were stockpiled and packed away.
The M1C/D rifles made an appearance briefly in the Korean conflict, but they were never widely used and after those hostilities came to a cease fire, the M1C/D rifles were again packed away. Then the Vietnam conflict broke out and the US found itself in a guerrilla style war with a huge demand for qualified snipers and sniper rifles. This time around, the M1D rifles that were still around were not only old and out dated, but their 30-06 chambering was no longer the standard and the M1 Garand rifles were no longer in service. The US Army dabbled with scoped M-16s with very little luck, so finally a serious effort was made to develop and adopt a proper sniper rifle.
The US Army began its search in the same year that the USMC did, 1966, but the US Army did not go the route of a bolt action sniper rifle like the Marines, instead, they were believers in the semi-automatic and they started with modifying the M14. The M14 itself was an adaption of the original M1 but chambered in 7.62x51mm NATO (308 Win) and fitted with a detachable box magazine (DBM). They first tried using the M84 scopes from the old M1D sniper rifles and mounting them on top of a M14 with a modified swing over mounting system. The rifles were tested with both M80 and M118 match ammo, but the results were not very positive. The 2.2x magnification of the M84 scope was deemed unsuitable, the accuracy of the rifle with M80 ball ammunition was also unsatisfactory, and the scope mount, a hinged unit, was ruled deficient. But the National Match version of the M14 with M118 match grade ammo proved adequate and worthy of further development. So they continued with the program.
Because of the pressing need for sniper rifles in Vietnam, the Army actually deployed a good number of standard M14s with M84 scopes mounted in various different ways. This was done until a suitable permanent replacement system could be developed for the Army sniper program.
In 1964 James Leatherwood came up with a clever design for a mounting system that would allow a scope to be automatically adjusted when the zoom ring was changed. This was done via a cam system that would physically tilt the scope forward as the zoom power was increased. When he was based at Ft. Benning Georgia in 1965 the concept was presented to the Limited Warfare Laboratory (LWL) and a project was started to design a system that would work on a sniper rifle. An all-aluminum system was subsequently designed and then tested in 1967. It was called the Adjustable Ranging Telescope (ART) and was combined with a Redfield Accu-range 3-9x40mm commercial scope. You will probably recognize that this is the same scope the USMC adopted for their M40s during the same time. It was considered the best scope on the civilian market at the time.
The US Army had the scopes made without the tombstone ranging mechanism inside the reticle and instead had the scopes installed with a reticle that had marks on the stadia that covered 30 inches at 300 meters on the vertical and 60 inches at 300 meters on the horizontal. Since the reticle was located on the 2nd focal plane and did not grow and shrink with the power setting, it could be used to provide range estimation from 300-900 meters. The sniper would just zoom in until the marks on the vertical crosshairs would cover 30 inches, typically the belt line to the top of the head, and then look at the marks on the power ring to determine the range. At the same time, the cam would actually already be raising, or lowering, the scope to automatically compensate for the range at that distance. The entire unit performed as a system for rapid and accurate target engagement. Zoom in, hold for wind, pull the trigger. The cams were made to match specific cartridges and there were three different cams that could be switched out, one each for the M118 Match, M80 Ball and M2 50 BMG ammo.
Of course, all of the fancy auto-ranging ability of the ART system would do no good if the rifle could not perform adequately as well. The LWL contacted the United States Army Marksmanship Training Unit (USAMTU) to have them put together an accurized M14 for use with the ART. The USAMTU is where all of the competitive shooters for the Army are stationed and they came up with an extensive list of modifications performed on a National Match M14 rifle that included the following:
- Disassembled down to the receiver
- Barrels were selected for straightness and uniformity
- Barrels were installed with minimum headspace on the chambers
- Area of contact for the rod guide was knurled to prevent the guide from rotating on the barrel
- Gas cylinder and band were screwed together as an assembly and internally polished to reduce carbon build-up
- Piston was polished
- Flash suppressor was reamed out to specifications shown to provide best accuracy with M118
- Barrel and flash suppressor were machined for a perfect alignment
- National Match stocks were treated in a vacuum sealed oven, baking at 300 degrees for an hour to remove all moisture from the wood, then the stocks were impregnated with epoxy and baked for an additional hour while held at 100 psi. Then the stock was placed in an oven to cure for 3 days. This was done to stiffen and eliminate any warpage or swelling due to weather changes.
- Stock liner was removed to provide 1/8” of bedding compound in the recoil areas
- Barreled actions were glass bedded in a two stage process to provide centering, and then pre-loading of the front part of the stock to dampen barrel vibrations.
- Triggers adjusted to 4.5 – 4.75 lbs
- Hand guard was cleared of the stock and anchored to the band
- Gas cylinder lock was indexed to be finger tight at the six o’clock position
- A new operating spring guide was installed
- Cams, corners, and bearing surfaces throughout the mechanism were modified to provide smoother operation and uniform return of all moving parts
- Rifles were then tested for accuracy and the ART mounted and zeroed
As you might imagine, these rifles were very accurate. They performed tests where they were getting 10 round groups at 900 meters that measured under 10”. The rifle was designated the XM21 The Army then sent 10 of these original XM21’s to Vietnam in 1967 for evaluation. There were several weaknesses found, but overall they were satisfied with the system as a whole. At the same time some M16s were being tested as sniping systems as well, but the smaller 5.56 cartridge could not match the long range capability of the XM21. The Army also considered bolt action rifles but elected to go with a semi-auto because it provided the ability to perform rapid follow up shots, rapidly engage multiple targets, allowed the sniper to better defend themselves and the M14 already was setup for night vision capability. They did recognize that the one major disadvantage was the flying brass that was ejected and could be a target identifier, but this was considered to be acceptable when compared to the advantages.
At about this same time the 9th infantry division requested help from the USAMTU in setting up a sniping program so in 1968 the AMTU sent over 10 instructors and then later sent over the first XM21 rifles for use with the 9th ID. Also, in early 1969 the AMTU sent over a batch of Sionics M14 suppressors for test and evaluation at the 9th ID sniper school with great success.
Finally in February of 1969 the XM21 was fully funded by the US Army and larger production began and the XM21’s were deployed in large numbers, about 50 per week being built to the exacting specs already mentioned. Officially the name remained the XM21 up until 1972 when the Army officially adopted it as the sniping standard and it became the M21. There were over 1300 XM21s used during the Vietnam conflict and while there were some failures, the rifles held up extremely well and served with distinction and great effect out to 900 meters. After Vietnam, the sniping program in the US Army went dormant, again, until some interest was rekindled in 1976. The M21 was tested against the M40A1, M82, AR10, and some others and the M21 more than held its own and the Army elected to hold on to it as is. Once the official Sniper program was adopted by the Army in 1988, the M21 was replaced by a new bolt action sniper rifle known as the M24. There are still some M21’s in Army vaults around the country, but none are in wide spread use by the US Army today. Though the M21 still lives on having spawned some very successful offspring such as the XM25 and EBR.
As you can see from the comments below, the M21 holds a dear spot in many U.S. Army snipers hearts (me included), and rightfully so.