AR-15 Sniper Rifles: Special Purpose and Designated Marksman

August 9, 2014 1:09 pm

The semi-automatic AR-15 based sniper weapon system, often referred to as a Special Purpose Rifle (SPR) or Designated Marksman Rifle (DMR), is a precision weapon ideal for close to medium ranges (25yd-800yd). By definition, a “sniper” rifle is simply a scoped rifle used to shoot at human targets. By dedicating the build of a rifle for this sole purpose, an AR-15 can become the top choice for a semi-automatic sniper system. The sniper or DM is able to deploy a highly accurate sniper rifle that fires the commonly found 5.56 or .223 ammunition. A traditional bolt-action sniper weapon is not ideal for clearing buildings and other close-in work that is common in today’s urban environments. Utilizing a match grade 20″ Stainless Steel heavy barrel with a 1/8 twist, while heavier than a mil-spec M4 profile 16” carbine length barrel, is still short and light enough that the operator can utilize the DMR in CQB engagements and clearing buildings while providing the ability to range targets out to 600+ yards using standard 55gr ammunition and 800+ yards when loaded with 69gr – 77gr match grade ammo. The heavier 20” SS barrel stabilizes the rifle better than that of a carbine length barrel allowing the operator to fire slightly faster and more accurately. The rifle length gas system of an SPR/DMR provides softer felt recoil and helps the shooter maintain his crosshairs on target while firing the rifle. The low recoiling combination of an SPR/DMR AR-15 firing 5.56/.223 ammunition also offers the advantage of faster followup shots when compared to larger caliber rifles, as the sight picture remains stationary and the shooter can observe hits through the scope.

The AR-15 based sniper rifle is also ideal as a spotter’s weapon. The spotter then has the defensive firepower of light semi-auto, with long-range capabilities to match his training should the need arise. The rifle can also be used as a backup for the sniper’s bolt-action rifle should it become disabled for any reason.

Accuracy from the AR-15 Platform.

The long range and varmint hunting crowd had discovered something that the military was slow to recognize; by an accident of design, the AR-15 rifle is an ideal platform upon which to build a highly accurate rifle.

In the AR-15 rifle, the barrel is threaded to the barrel extension which functions as the receiver on a traditional rifle. Headspace is set between the barrel and the barrel extension. The bolt locks into the barrel extension when it is in battery, and this group of parts lock together to contain the cartridge in the chamber and guide the bullet toward the target.

In any rifle, accuracy is largely the result of the perfect alignment of the bolt, barrel, and receiver as well as the quality of the barrel. Interference with the barrel is generally detrimental to accuracy. Much time and expense is spent on truing and matching the mating surfaces of these parts when tuning the accuracy in a bolt rifle.

In the AR rifle, the upper receiver is simply a housing for the bolt and carrier and an attachment point for the barrel. The barrel, barrel extension, and bolt are locked together. The upper receiver touches only the barrel extension where the barrel is held in place by the barrel nut. The bolt floats in the bolt carrier, which floats in the upper receiver. This allows the bolt to align itself perfectly with the face of the barrel.

The beauty of this design is that free-floating a quality barrel in an AR-15 rifle results in accuracy that rivals very expensive finely tuned bolt-action rifles.

Precision Sniping Requires a Free-Floated Barrel.

Take a rifle with a military barrel contour, hold the receiver so that it cannot move, and pull down on the barrel with one finger. You will be shocked to see how much the barrel moves. If the barrel moves while shooting with iron sights, the front sight moves as well and will partially compensate for the variation. If optics are employed, the variation will be much more evident downrange. Some sniper instructors did a demonstration while training designated marksmen that really drove these points home. They took a good shooter and had him zero his rifle resting on the magazine. He then shot a tight group on a target at 100 yards from the magazine rest. They then placed sandbags and had the shooter rest the forearm on the sandbag. The group fired from the sandbag rest was several inches higher on the target at only 100 yards.
The obvious solution is to replace the handguards with a free-floating system. Several companies offer float tubes for service rifle competitors that are almost indistinguishable from military issue configuration.

If replacement with a rail system is an option, there are several systems that can free-float the barrel without gunsmithing. The rails are removable, so only necessary rails need to be installed in any location around the handguard. If a gunsmith is willing to take a hacksaw to the delta ring and weld spring, a two-piece design such as a Midwest Industries Two-Piece Free-Float Hangaurd can be installed without removing the front sight tower and flash hider.

If a bipod is used, it should always be attached to the free-floating handguards and not to the barrel.

Adapting the AR-15 for long-range shooting.

There are several things that need to be taken into consideration when building a long-range SPR-15 / DMR-15. In short summary, the correct free-floated barrel, optics, ammunition, and trigger are the most important factors. Second to the basics would be a proper buttstock, free-float tube and bi-pod.

Choosing the Correct Barrel for a DMR.

As mentioned earlier, a 20” SS match grade heavy barrel chambered for 5.56 is primarily used when building a DMR-15. SPR-15s are essentially the same rifle as a DMR but are often found utilizing an 18” mid-weight barrel. It is important to stick with a Stainless Steel barrel for precision shots over 400 yards. The heavier the barrel, the more rigid it is. Rigidity plays a large factor when balancing the barrel’s harmonics and reducing any barrel sway when firing.

Specific Optics Are Required to Best Utilize the Full Potential of an SPR/DMR.

For a Special Purpose Rifle or Designated Marksman Rifle, the ideal scope to mount is one capable of 1x-8x or 2.5x-10x magnifications. At 1x the optic works basically as a reflex sight. Adjusting to a higher magnification allows more precision for longer ranges or smaller targets. Companies such as Leupold, Schmit & Bender, Vortex, Valdada IOR, Millett, and Burris make this type of optic. These optics are very nice and provide great flexibility to the DMR. Millett’s DMR-1, while not a Schmitt & Bender by any means, provides solid quality for the price. Optics are clear and it works well at all magnifications. It seems that most of the optics with ballistic drop compensating reticles for the 5.56 are designed for the M855 or other cartridges with similar trajectories.

For dedicated long-range shooting with a DMR more so than an SPR, more traditional scopes usually work if mounted as far forward as possible in standard rings. The charging handle generally will end up under the scope in this case and an extended charging handle makes operation easier. Backup iron sights will rarely fit under the scope and would require removal of the optic if they did. Magpul has released an enhanced version of their MBUS flip up iron sight that sports a lower profile for use with low mounted scopes. Another option would be a reflex sight mounted to the scope. If possible, utilize both options for added redundancy incase optics become severely damaged while in the field.

Aftermarket Triggers Enhance Accuracy.

The standard mil-spec trigger found on any AR-15 is rough, gritty, single stage, and usually requires approximately 8lbs of force to fire the weapon. These triggers are not intended for mid range precision and sniping. The force required to pull a mil-spec trigger often jerks the rifle just a cunt hair ..but at 600+ yards, that tiny twitch can throw your shot far off target. To build a proper SPR or DMR, a 2-Stage 2.5lbs – 3.5lbs match trigger is required for precision sniper shots.

Ammunition Type May Be The Single Most Important Factor in Mid Range Sniping

The 5.56 cartridge has had a reputation of poor to mediocre performance in combat though it deserves a much better reputation than it has. Disappointment with the 5.56 is mainly due to the type of ammunition fielded.

If a 5.56 projectile passes straight through a person, it makes a very small wound channel. Unlike larger calibers, for the 5.56 to kill effectively, it must fragment. If the 5.56 fragments well enough and early enough, it is quite effective. Fragmentation requires a properly designed projectile and sufficient velocity to cause fragmentation.

During the cold war, the U.S. military developed the 62gr M855 cartridge. It was designed to be fired from a long barreled SAW at a husky potato-fed Soviet soldier wearing a flack jacket. Today our soldiers are firing that cartridge from 14.5″ barrels at 95 pound insurgents wearing man-dresses and durka durka towels. The average target for US soldiers today is 7.3″ thick at the chest. The M855 averages 7″ of penetration before beginning to yaw or fragment. The result is small ineffective wound channels. Multiple hits are required to eliminate threats. From an M4 barrel, the velocity drops to the point that fragmentation often does not occur past 100 yards.

Fortunately, there are better options out there. The Mk 262 mod 1 ammunition, manufactured by Black Hills Ammunition, is used by the military for sniping applications and fires a 77gr open tip boat tail Sierra Match King bullet at ~2800ft/s from a 20″ barrel. This load is effective for hits out to 800 yards. A civilian version is offered by Black Hills that is safe to fire from .223 chambered rifles though its muzzle velocity averages a little slower than the military version. Hornady manufactures a 75 gr TAP cartridge that provides similar terminal performance. Both Mk 262 and Hornady TAP cartridges begin fragmenting at about 3″.
Black hills now has incredible 55gr and 62gr TSX cartridges that expand reliably to an average of .45 inches, and tend to maintain about 100% of their weight for deep penetration. It is an excellent intermediate barrier penetration cartridge, with excellent expansion and weight retention after penetrating vehicles or windshield glass.

These cartridges are in use by the US military and are used exclusively by many in the special operations community who have found them to be far more effective than standard M855 ammunition. They are excellent choices for both law enforcement and self defense roles.

A Good Buttstock Makes a Big Difference.

Important considerations are durability and simplicity. There are very expensive stocks out there that have buttons and wheels and really cool looks that would require three hands and an engineering degree to operate. Everyone also seems to want collapsible sniper stocks with adjustable cheekpieces. A shorter stock and higher cheekpiece make little sense for the rifles we are discussing here. They also work against each other: the shorter the stock, the shorter a cheekpiece must be for the charging handle to clear. I know from experience that a collapsible buttstock on a scoped AR-15 never seems long enough even when wearing body armor. I like a little more length on a buttstock and a rubber pad to grip body armor when necessary. An adjustable cheekpiece is ok to adjust for different shooters as long as it clears the charging handle.

The standard A2 buttstock, while longer than ideal for a tactical rifle, works well for a scoped rifle. Two stocks worth mentioning are the Magpul UBR (Utility/Battle Rifle) and the Magpul PRS (Precision Rifle/Sniper). Both Magpul stocks have adjustable length of pull but are longer than standard collapsable stocks. They are also fairly heavy for those who wish to add weight for recoil reduction and to better balance the weapon due to the heavier 20” barrel.

Free-Float Handguards and Bipods Are a Must Have.

There are an overwhelming variety of free-float handguards available. Choosing something with only the necessary rails for the equipment to be mounted will shave unnecessary weight and bulk. “Smooth Tubes” with 15 inches of length are ideal for the rifle length gas system as they clear and protect the low-profile gas block as well as provide the correct distance to mount a bipod.

As with handguards, bipod designs are becoming more numerous and expensive. Many of the newer designs are very awkward to use and have no real advantage over traditional designs. Both the Versa-Pod and the Harris bipods work very well. There are adaptors available to mount Harris or other sling swivel mount bipods to a Picatinny rail.

For the designated marksman who wants more flexibility in his weapon system, there are several forward vertical grip designs that incorporate a built in bipod of some kind. Most of these bipod grips lack any adjustment and are very unstable. Some of those that allow cant and pivot are even more unstable. The rifle will easily fall over if the shooter lets go for any reason. There are a few companies that produce sturdy grips which split in half to become a traditional style bipod with fully adjustable legs. Some vertical grip bi-pods contain a built-in 120 lumen LED tactical light to combine three essential accessories in one compact package.

SPR-15 vs DMR-15

The primary difference between these two sniper rifle builds comes down to length and weight. A Special Purpose Rifle (SPR) will almost always sport an 18” mid-weight barrel while the Designated Marksman Rifle (DMR) will utilize a heavy-weight 20” barrel. Both rifles will handle CQB and medium range sniping though the SPR carries a slight advantage for CQB due to shorter length and less weight while the DMR leans towards the other direction with less recoil and slightly higher muzzle velocities. Both AR-15 sniper rifles will get the job done’s just personal preference for the environment where one will be advancing or defending.

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