April 27, 2021
By David M. Fortier, Senior Field Editor
The Mk12 SPR series is an interesting example of how much can be done with the basic AR-15 design. The 5.56mm Mk12 Mod 0 and Mod 1 both proved very effective when fielded in combat in both Afghanistan and Iraq. In one engagement in Afghanistan two US Special Forces troopers armed with Mk12 Mod 0 rifles killed 93 Taliban insurgents. Accurate and effective, the Mk 12 series combined proven concepts from both the military and competition shooting. So, I thought it would be interesting to look at the Mk12’s development and examine the original Mod 0. The rifle seen on these pages is an accurate replica built by the Firearms News Gunsmithing Editor, Gus Norcross. Norcross served as an NGMTU armorer and did an excellent job on this build.
The concept for what later became the SPR was imagined by ArmaLite, Inc.'s President, Mark Westrom. He felt a light, accurate, optically sighted AR capable of precision fire would be a handy item in a fight. He referred to it as an SPR, or Special Purpose Rifle. While Mark conceived the idea, it wasn't until much later that the US military actually became interested in the concept. Not until the 5th Special Forces Group saw the need for a light and compact weapon which could provide both precision and support fire was the idea dusted off. 5th Group's original thought was for an upper receiver assembly which could be swapped onto existing M4A1 carbine lower receivers. This would allow any M4A1 to be easily transformed by simply swapping upper receiver assemblies. Rather than being a Special Purpose Rifle, as Westrom conceived it, 5th Group was interested in what they referred to as a Special Purpose Receiver. And so the project began.
The initial draft of the desired requirements was made by SOPMOD Programs Office at the Naval Surface Warfare Center in Crane Indiana. Starting in late 1998 5th Group began working with the US Army Marksmanship Unit on the project. This included the development and testing of several prototypes. This initial testing proved the worth of the concept, and in October of 1999 the SPR was validated as part of the SOPMOD requirement.
Following this, work began on developing and refining the concept into a fieldable design. Testing began to determine both the optimum barrel length and profile. Along with the standard 20-inch length, prototypes were also built with 18 and 22-inch barrels. To enhance accuracy a high-quality Match grade barrel, rather than a standard Mil Spec tube, would be fitted. Along with different barrel configurations, different models of free-floating handguard tubes were also tested. These were intended to free-float the barrel to enhance accuracy, provide a secure surface to grasp, protect the operator's hand from heat build-up, and to provide a foundation for mounting accessories such as a sling and bipod.
As the weapon was intended to be primarily utilized with an optical sight, the upper receiver was in the flattop configuration. To enhance the mounting of optics, a number of different types of extended rails were tested. Some of these ran the full length of the receiver and handguard, and some simply extended forward of the receiver. A compact tactical scope developed by Leupold for this application would be mounted. Back-up iron sights were to be mounted in case the optic failed. These were intended to fold neatly out of the way when not required. In an emergency though, they could be easily flipped-up and targets engaged in a conventional manner.
The first batch of upper receivers were assembled at NSWC in Crane Indiana using 18.5-inch barrels from three different manufacturers. Of the 150 upper receivers assembled 50 were built using Douglas Match barrels, 50 using Krieger Match barrels and 50 using Schneider Match barrels with polygonal rifling. These uppers sported aluminum free-floating handguards and were fitted with A.R.M.S. #38 Swan sleeve rails. While extremely accurate, testing revealed one flaw in the concept. The trigger pull of a standard M4A1 upper was heavy and gritty enough that it was detrimental to the weapon's practical accuracy. To cure this Match grade triggers (still capable of full automatic fire) needed to be installed into the lower receivers. Due to this, the concept of a universal drop-on upper receiver able to be mounted onto any M4A1 was unable to be fulfilled.
To rectify this the term SPR evolved from Special Purpose Receiver to Special operations forces Precision Rifle. While it was originally envisioned building the upper receivers into complete rifles using M4A1 lowers, these proved to be in short supply. What Crane did have in abundance of though were M16A1 rifles. These were being turned in by National Guard units for destruction. As many of these were in excellent condition, it was decided to use an M16A1 lower receiver as the foundation for the rifle. This would provide a readily available receiver capable of semi-automatic or full-automatic fire.
Formal testing of the first SPR rifles began in October of 2000. Then over the winter some final design changes were implemented and 100 Limited User Test rifles were built. The attack on the United States on September 11, 2001 led to most of these initial 100 LUT rifles seeing combat in Afghanistan. During combat operations, US Special Operations Forces so equipped accounted for a surprisingly large number of enemy fighters killed in action. In many instances, the sheer number of enemy fighters killed by SPRs is staggering and slightly reminiscent of the Eastern Front during World War II. Without a doubt, the little 5.56 precision rifle was a success. Its successful deployment eventually led to Crane type classifying the SPR as the Mk12. This series consists of two models, the Mod 0 and the Mod 1. Next, we will take a closer look at the original Mod 0.
The Mk12 Mod 0
The heart of this rifle is a M4 flattop upper with its extended feed ramp and special barrel extension. To this is mated an 18-inch long Douglas Match barrel manufactured from 416 stainless steel with 6 groove rifling and a 1-7 inch right hand twist. From what I understand, Crane settled on Douglas barrels for their accuracy, rapid break-in, durability and price. These were turned to a specific contour by a barrel maker (who will remain nameless) well-respected among competitive riflemen.
In place of the standard A2 flash suppressor is a OPS, Inc. muzzle brake. This is threaded externally to accept Phil Seberger's 12th Model SPR suppressor. To the rear of the brake is a barrel collar precisely fitted to a step in the barrel. This collar aligns and tensions the suppressor. With the suppressor mounted accuracy is as good, or better, than without the suppressor. Performance of the can is impressive, and it carries a 40 dB reduction rating.
In place of the standard gas block/front sight housing is a unit from Precision Reflex, Inc. This incorporates a folding front sight assembly which folds neatly out of the way when not in use by simply pushing a button. The protected front sight is adjustable for elevation when zeroing.
Surrounding the barrel is a carbon fiber free-floating handguard also made by Precision Reflex. This has short rails mounted at 3, 6 and 9 O'clock to allow mission essential accessories to be easily mounted. On the 6 O'clock rail an A.R.M.S. QD bipod adapter is attached which allows a Versa-Pod bipod to be easily mounted.
Running along the entire length of the upper receiver and handguard is an A.R.M.S. #38 SPR PEQ-2-3 Swan sleeve. A heavy and robust unit, this features MIL-STD-1913 cross slots to allow day and night optics to be mounted anywhere along its length. At the rear of the sleeve is an A.R.M.S. #40 Stand Alone Flip-Up rear sight. This simple and rugged rear sight is adjustable for windage and folds neatly out of the way when not required. By disengaging its release lever it pops up ready for use. Sitting just below the rear sight assembly is Precision Reflex's Gas Buster charging handle. This unit is fitted with an extended latch and is designed to prevent gases from the rifle's interior from reaching the shooter. This is an important feature when utilizing the sound suppressor.
While some preproduction guns were equipped with Leupold LRM3 rifle scopes, the standard optic is Leupold's TS-30A1 or A2 scope. These are 3-9x36mm Tactical scopes which Leupold introduced commercially as their Mark 4 Mid-Range/Tactical. Actual magnification on this model runs from 3x to 8.7x. Objective size is 36mm. Equipped with M3 type turrets, the elevation knob is delineated in 1 MOA clicks and also features a Bullet Drop Compensator. Windage adjustments are in .5 MOA clicks. This optic is 11.3 inches long and weighs 16 ounces. The only difference between the A1 and A2 models is the A2 features an illuminated Mil-Dot reticle. The optic is mounted to the weapon via a pair of A.R.M.S. #22 medium rings. These allow the optic to be easily removed from the weapon via a QD throw lever system.
The upper receiver is fitted to a modified M16A1 lower receiver. These units were stripped and rebuilt using a Knight's Armament two-stage selective-fire Match grade trigger. In the semi-automatic mode this trigger is designed to act as a conventional Match trigger, and provides a crisp and clean release. This allows a trained shooter to take full advantage of the rifle's accuracy. Yet, it is still fully functional in the Auto mode, allowing it to be used as a light support weapon.
A2 buttstocks were mounted and many (but not all) rifles were equipped with Ergo grips from Falcon Industries. In addition, many rifles were also equipped with ambidextrous selector levers. The result is a 38-inch long rifle weighing 11.7 pounds (with sound suppressor) which is fairly compact, maneuverable and capable of fine accuracy.
Feeding the Mk12
Now, a precision rifle is only as good as the ammunition you feed it. The accuracy criteria for standard 5.56x45mm 62-grain M855 ball ammunition is a maximum dispersion of 4 MOA from 100 to 600 meters. Obviously something better was needed. So, from the start the Mk12 was intended to fire dedicated ammunition. As Black Hills Ammunition was producing extremely high-quality 5.56mm Match ammunition for all the Armed Forces Rifle Teams, they were contacted about the project.
Testing was undertaken using a variety projectiles and powders with the goal being for enhanced accuracy and terminal performance at extended distances. At first a 73-grain Berger Open Tip Match bullet was selected, but this was later changed to a 77-grain Sierra MatchKing. This was loaded into a military case with a crimped primer. Rather than being loaded to commercial .223 Remington pressures, this ammunition was loaded to higher 5.56mm NATO pressures to enhance performance. The resulting load was very similar to Match ammunition loaded for the Army Marksmanship Unit for use in competition.
The Mk12's ammunition evolved and was eventually type classified as Mk 262 Mod 0 and Mod 1. The primary difference between the two types is the addition of a cannelure to the Mod 1 projectile to prevent bullet set-back during feeding. Both terminal performance and accuracy of this ammunition is markedly improved over M855 ball. Each lot is tested for accuracy by firing ten 10-shot groups at 300 yards. The average group size is between 2-2.5 inches. Unlike the 62-grain M855 ball round, the 77-grain Sierra MatchKing fragments much more reliably out to much longer distances. This dramatically increases terminal performance. The downside is inferior penetration to the M855 round.
On the Range with a Clone
Just to give an idea on the Mk12 Mod 0’s performance, we hit the range the clone seen here. Firing from the bench at 100 yards Black Hills Ammunition’s 77-grain Match load put four rounds into .42-inch and all five into .6 inch. Switching to Black Hills Ammunition’s Mk 262 Mod 1 77-grain OTM military load it again put all five into .6 inch. Most impressive though was the velocity of this 5.56mm pressure load. The 77-grain Sierras were averaging 2,783 fps from the short 18-inch barrel. Shooting at 300 yards on a different day the Mk12 Mod 0 put five rounds of Hornady 75-grain TAP T2 into 2.5 inches. I've been told by an individual involved in building the Mk12s for the military that the guns are capable of shooting into 8-inches at 600 yards.
As to the rifle's actual combat performance, let me share a brief story. A 5-man Special Forces team looking for Scuds in Iraq was attacked by a reinforced Iraqi infantry company. Three men with Mk12 SPRs covered the unit’s retreat back to the LZ. In doing so they killed 167 Iraqi soldiers, with the rest either running away or surrendering. Performance like that is hard to argue with. The down-side to the Mk12 series is its weight VS exterior ballistics. While compact, it is a very heavy system considering its caliber. Yes, it is quite accurate but as the ranges move past 500 yards the 5.56mm cartridge begins to run out of steam. Ultimately while the 5.56mm cartridge can reach out to distance, it is not ideal for this. There were also problems with handguards getting damaged during rough handling. In time, better solutions became available. Even so, the Mk12 series remains an interesting design and piece of American military firearms history.
If you have any thoughts or comments on this article, we’d love to hear them. Email us at FirearmsNews@Outdoorsg.com
About the author:
David M. Fortier has been covering firearms, ammunition and optics for 23 years. He is a recipient of the Carl Zeiss Outdoor Writer of the Year award and his writing has been recognized by the Civil Rights organization JPFO. In 2007 he covered the war in Iraq as an embedded journalist.
Specification Mk12 Mod 0
Caliber: 5.56x45mm NATO
Barrel Length: 18 inches
Barrel Twist: 1-7 inches
Weight: 10 pounds, 11.7 pounds with sound suppressor
Overall Length: 37.5 inches
Feed: 20 or 30-round detachable box magazines
Effective Range: 700 meters