The Mk12 SPR (Special Purpose Rifle)

The Mk12 SPR (Special Purpose Rifle)

ABOVE: The Mk12 SPR has seen significant amount of action in the Global War on Terrorism being very popular with the Special Operations operators who use them. They have proven themselves to be a very lethal rifle with the Mk262 Mod 1 ammunition that was designed for it. It is not unusual to get 100% hits at 850 yards on human silhouette target in the hands of a properly trained sharpshooter.

The accuracy of the AR-15/M16 has always been top rate. For the longest time, when one would think of a long range designated marksman rifle (DMR) you would think of something in 7.62x51mm NATO and more than likely bolt action. During the Vietnam War, it was seen that there is a benefit to having a semiautomatic DMR-type rifle so the accurized M14 appeared. In the calm of the Cold War, little attention was paid to small arms; money was going into nuclear weapons and advanced aircraft. Since the Vietnam War, U.S. troops encountered little combat and small arms would remain virtually unchanged. After 9/11 all that would change. Now with the Global War On Terrorism, infantry battles would be back rather than the high tech air war, which really was the First Gulf War. Prior to that, former Army Colonel Mark Westrom, former President of ArmaLite, Inc. conceived a 5.56x45mm SPR (Special Purpose Rifle); originally envisioned by Westrom to be in 18 inch, 20 inch and 22 inch barrels. The SPR as initially designed was to be an upper receiver that would be adaptable to current M4/M4A1 carbines that would fill two roles. First as a light sniper rifle and then, if need be, it could be used as a light machine gun. There was again no one rifle or carbine available that would fit this particular role so SOCOM would build it. Unfortunately it stopped, at least for a short period of time.

The concept sat dormant for years until SOCOM revived the concept in the 1990s as an initiative by the 5th Special Forces Group. They envisioned the SPR as a Special Purpose Receiver that drops on a standard M16/M4-type lower receiver. This receiver was to be highly accurized and would shoot a new type of 5.56x45mm round – one that would go on to be the most accurate 5.56mm cartridge in the world. The SPR upper receiver would provide a lightweight, compact, long-range precision fire and light support capability to the small Special Operation Forces groups that were not in a position to receive support from aircraft or artillery.

Modified M12 Mod 1. It has the fixed A1 stock replaced with an LMT SOPMOD stock with a six position receiver extension. Although the MK12-series SPRs all came with fixed stocks from Crane, Indiana, once the rifles went to their assigned units modifications were made by the users. Also the Ops, Inc. sounds suppressor is attached. Note the Harris bipod.

SOPMOD Programs Office at the Naval Surface Warfare Center, Crane, Indiana, drafted the requirements desired and went to work soliciting and testing the concept at hand. In late 1998 and throughout 1999, the 5th Special Forces Group collaborated with the U.S. Army Marksmanship Unit (USAMU) at Fort Benning, Georgia, to develop initial prototypes of this new requirement. During the program, SOPMOD funded several prototypes that were made and tested by the USAMU working closely with the 5th Special Forces.

The first SPR prototypes were tested with handloaded ammunition and the concept was solidified with the requirements being realistic and achievable. Several match grade projectiles were tested with weights as high as 88 grains. After rigorous accuracy testing, the 73-grain boat tail match open tip bullet manufactured by Berger was chosen for the SPR program. However, due to the immediate requirement for ammunition to go along with the SPR upper receiver, the Berger factory was in the process of moving and could not deliver the bullets in the needed quantities. With this kink in the chain, a new bullet was needed to fit the requirement. The new bullet would be the Sierra 77-grain Boat Tail Match King. Jeff Hoffman, president of Black Hills Ammunition was charged with designing and producing the ammunition that would both maintain match accuracy and combat reliability of theM16A2 rifle. Based on these findings, in October of 1999, the SPR was validated as part of the SOPMOD requirement. First requirements called for the drop-in SPR for the M4A1 carbine along with the match grade ammunition. Conceptually, there would be a modified M4A1 carbine that would have precision match grade accuracy in a lightweight rifle that could provide semi- or automatic firepower on demand. The rifle is designed as a match grade rifle but if the need would arise the heavy barrel would provide fully automatic suppressive fire.

Finalized MK12 Mod 1 rifle. Note the M16A1 lower receiver that would be equipped with a Knight’s Armament two stage selective fire trigger group. This rifle differs from the Mod 0 with the handguard and backup sights. The handguard used is the Knight’s Armament M4 Match Free-Floating Rail Adapter System (KAC part number 99167) and the KAC rail fore end flip-up front sight (KAC part number 99051). The Mod 1 also uses the KAC 600 meter flip up rear (KAC part number 98474).

All first production SPRs were assembled at NSWC Crane, Indiana. The first 150 receivers utilized 18 ½ inch barrels manufactured by Krieger (50), Douglas (50) and Snider (50 polygonal). Interestingly, the initial rifles used a 20-inch barrel. When it came time to procure, the Navy would not procure a 20-inch barrel because one already existed – the 20-inch M16A2/M16A4 barrel. So a new barrel could be procured to conform with the requirements. The length was reduced to 18.5 inches and that was ok to procure due to it not already being in the inventory. The barrel would use a standard rifle length gas system. The Douglas barrels, primarily based on performance as well as cost, became the barrel of choice. These barrels were attached by an aluminum free-floating handguard to an ArmaLite flat top (M1913) rail upper receiver. Added to the upper receiver was the Swan #38 sleeve rail or Swan Sleeve. SOCOM found that the ArmaLite as well as Colt receiver was approximately .005 too tall and caused some problems. To keep parts commonality with parts available in the supply chain, SOCOM eventually went with as many existing parts in inventory as possible. There were two main triggers in use. Due to the fact the SPR would have to perform as both a sniper rifle and a light machine gun, a match grade trigger was required but also with the capability to fire fully automatic. The first trigger implemented was the Knight’s Armament Company two stage selective fire trigger and the second was Accuracy Speaks single stage trigger. Based on research, it appears that the last production rifles used the Accuracy Speaks single stage selective fire trigger group.

When the MK12 was finalized, the acronym would remain the same but what it stood for would be different. SPR originally meant Special Purpose Rifle; however now it would stand for SOF Precision Rifle or Special operations forces Precision Rifle. Within 12 months, Crane took all their gained knowledge and used it to develop 24 second generation prototypes that were more adaptable to production on a larger scale. As originally envisioned, the upper receiver was designed to be dropped into the M4A1 lower receiver. For any number of reasons, this really was not a good idea. By building a dedicated rifle, it could be designed as an accurized rifle – for instance having a match trigger installed and a longer stock that would be more comfortable than the telescopic stock of the carbine. The host weapon would be the older and outdated M16A1 rifle due to the lack of availability of the M4A1 carbine at the time and a large number of obsolete M16A1 rifles were being turned in to Crane by National Guard and Reserve Units for destruction. In order to achieve the full performance requirements of the SPR, more was needed than just a drop in upper receiver.

A: On the left is the Sierra MatchKing 77gr OTM as first used in the Mk262 Mod 0 ammunition compared to the current Mk262 Mod 1 load on the right with the addition of the cannilure. B: Ballistic gel tests of the Mk262 Mod 1 ammunition provided by Jeff Hoffman, President of Black Hills Ammunition. C. On the left is the early Mk262 Mod 0. Note the 77gr Sierra OTM projectile. On the right is the current Mk262 Mod 1. Note the addition of the cannilure to the 77gr projectile. There were several improvements made to the ammunition from its initial to current load.

In October of 2000, formal testing of the first SPR rifles began at Thunder Ranch in Texas. Combination of operational and technical experimentation showed the remaining weakness that would be corrected before the production run of the second generation SPR rifles. During winter of 2000 and 2001, all the final changes were made and deficiencies were corrected. The first 100 Limited User Test (LUT) was set for large production runs. The plans called for these LUT rifles to be deployed with SOCOM operators overseas by the summer of 2001. These initial deployments allowed the users to evaluate and make suggestions for improvement before the final production run. Most of these initial 100 rifles were called into service due to Operation Enduring Freedom in September 2001 so the field trials were conducted in just that, the field. The SPR has been used with great success with Special Operations Forces engaged in combat in Afghanistan. The SPR is responsible for an extremely high percentage of enemy soldiers engaged and killed with precision rifles. The SPR rifle was now to be named, the Mk 12. There would be two basic models of the Mk12, the Mod 0 and Mod 1.

The Mk 12 Mod 0 and Mod 1 use the same lower receiver. The M16A1 lower receiver is fitted with a selective fire match grade trigger; the Knight’s Armament 2-stage selective fire trigger or the Accuracy Speaks single action trigger. The rifle is designed as a precision shooting rifle, however if needed, with the flip of the selector, the rifle can put down a heavy volume of fire. The barrel would be done as a match barrel after heavy automatic fire but that can be easily replaced if a high volume of fire was needed to save lives. The rifles may or may not be found with the ergo-grip manufactured by Falcon Industries, or with the standard A1-style pistol grip. Many of the lower receivers will have ambidextrous selector levers as well as ambidextrous Norgon magazine catches. Once rifles got to their units/end users, they were also customized for the unit or end user. Various pistol grips may be found and also seen has been telescopic stocks on Mk12 rifles. Due to the rifle length gas system and shorter 18-inch barrel, along with the heavier profile of the barrel, it was found the Mk12 would not run reliably on full automatic with a carbine buffer but would with the standard stock and buffer. However, some felt the shorter stock was worth the tradeoff of difficulty with automatic fire. During research, photographs have been found with Mk12 rifles built on M16A2 lower receivers as well with the 5/8 of an inch longer stock.

Finalized MK12 Mod 0 rifle. Note the M16A1 lower receiver that would be equipped with a Knight’s Armament two stage selective fire trigger group. The rifle uses Precision Reflex carbon fiber free floated handguard with a A.R.M.S., Inc. Swan #38 SPR sleeve and rear sight as well. The front sight is Precision Reflex and the Harris bipod is attached via A.R.M.S. #32 bipod adapter.

The upper receivers for the SPR/Mk12 series rifles utilize the standard flat top upper receiver with feed ramp cuts for use with a barrel extension also cut with extended feed ramps. Early production rifles would use a standard M16A4 flat top upper receiver (no extended feed ramps) and the feed ramps would be cut into them with a Dremel tool. The feed ramps were necessary so the thin jacketed 77-grain MatchKing bullets would not be damaged while feeding. The upper receivers in use are mostly produced by Colt that also includes upper receivers made by Diemaco/Colt Canada. Colt purchased Diemaco May of 2005 renaming the company Colt Canada. Prior to this acquisition, Colt purchased Diemaco manufactured upper receivers for both production M4 carbines as well as spare parts.

In the development stages, three manufacturers of barrels were used and tested; these would be Douglas, Kreiger and Schneider. In final selection, the Douglas barrel was chosen for a combination of accuracy, quality and cost. The barrel is made of high quality 416 stainless steel. This barrel utilizes a 1 turn in 7 inch twist with six lands and grooves and a right hand twist. The 1/7 twist was necessary to stabilize projectiles from 77 to 100 grains (subsonic). The end of the barrel has an Ops, Inc. muzzle brake, which has threads for mounting the silencer. The silencer is made by Ops, Inc. as well. Both incorporate Harris bipods. The rifles may also be found with various sound suppressors. The muzzle brake was very effective but also very loud. Due to complaints about the loud muzzle blast a screw-on flash suppressor was designed. This makeshift flash suppressor slid over the muzzle brake and screwed onto the threads. The concept was excellent and worked well but never really made it out of the prototype stages.

Update done to the Mk12 Mod 0 rifles by Precision Reflex, Inc. in July of 2007. Note the shorter 16-inch barrel and the rail mounted front sight. The rifle has the PRI Gen III forearm and ACE M4 SOCOM stock.

Both versions of the rifle utilize the PRI made Gas Buster charging handle, which is designed to prevent any gas from the upper receiver exiting out of the rear. The Gas Buster charging handle seals the rear of the receiver so the shooter will get no gas in his face. Some operator were known to put rubber ATV around where there were gaps in the fit between the charging handle and receiver to further seal that area from escaping gas.

Both rifles utilize the same main optical sight: the Leupold TS-30A1 and the TS-30 A2. Both are a 3x to 9x variable scope. The A2 model has the option for the operator to use an illuminated reticle. The intensity of the light may be adjusted by the knob on the top rear of the scope. This was the standard optic but you will encounter numerous types of optics in use.

The MK12 Mods 0 and 1 are designed for the Ops, Inc. sound suppressor. In order to accept the suppressor, the Ops, Inc. muzzle brake and suppressor collar are installed on the barrel. On the bottom was an experimental flash suppressor attachment. It was screwed onto the muzzle brake converting the muzzle brake into a flash suppressor. Many users requested this due to the extreme muzzle flash seen with the use of the muzzle brake. It never went to production but was an excellent solution to the problem at hand.

The Mk 12, Mod 0
NSN 1005-01-504-3275

The Mk 12, Mod 0 is carried by the U.S. Army Special Forces Rangers. The Mod 0 has a weight of 11.70 pounds. The major difference in the Mod 0 and Mod 1 is the handguard assembly and the back-up sights. The Mod 0 uses the A.R.M.S., Inc. #38 SPR Mod. Swan Sleeve with the PRI (Precision Reflex Industries) Ged III Freefloat Forearm, which is made from aluminum and carbon fiber materials. The Swan Sleeve goes from the handguard and covers and protects the rail on the upper receiver. At the rear of the rail is the SWAN #40 Stand Alone Flip-Up rear sight. The gas block is made by PRI and has a folding front sight. The front sight post is adjusted for elevation by a dial on the front sight assembly. If the optic was lost, by turning the two throw levers on the scope mount, the scope can be removed and both back-up sights can be engaged and the rifle will be ready for action.

The optics (light sources, bipod, etc.) are attached by A.R.M.S., Inc. throw lever mounts that allow for quick detachment if there is an immediate need to go to iron sights. For scopes, the throw lever mount #22M is used. Both Harris bipods as well as Versa-Pods are used. The upper receiver provided for this article was one of David Dunlap’s original uppers he built towards the beginning of the project. David Dunlap is the President of Precision Reflex, Inc.

The front sight base of the MK12 Mod 0 contains a folding front sight that is adjustable for elevation only.

In July of 2007, Precision Reflex, Inc. rebuilt 12 Mk12 Mod 0 rifles at Fort Campbell, Kentucky. These rifles deviated from the standard Mod 0 in 8 ways:

Replaced the barrels and bolts with a 16” Douglas, 1:8 twist 5.56 barrel – PRI #06-681BB
Removed the barrel mounted front sight with a PRI Rail Mounted Flip Up Front Sight – PRI #05-0028
Added a new gas block with intermediate length gas tube – PRI #05-075-01
Replaced the forearm with a new PRI Gen III Rifle Length Forearm – PRI #05-073-03
Replaced the old OPS Brake and sleeve with a New Ops Brake and Sleeve
Added a new ACE M4 SOCOM stock
Added an Accuracy Speaks full auto trigger
Replaced the Old PRI Gas Buster Charging Handle with new PRI Gas Busters – PRI #05-0031. (Old ones quickly “disappeared” to other users)

The rear sight for the MK12 Mod 0 is the A.R.M.S., Inc. #40 backup sight that has both long and short range apertures and is adjustable for windage only. RIGHT: The Mod 1 also uses the KAC 600 meter flip up rear (KAC part number 98474), which is adjustable for windage only. OPPOSITE PAGE TOP: The Mod 1 uses the KAC rail fore end flip-up front sight (KAC part number 99051) which is adjustable for elevation only.

The Mk 12, Mod 1
NSN 1005-01-504-3276

The Mk 12, Mod 1 is carried by U.S. Army Rangers, U.S. Navy SEALS and the U.S. Air Force Special Tactical Teams. The Mod 1 has a weight of 10.80 pounds and uses many of Knight’s Armament Company’s (KAC) components. The Mod 1 uses the KAC Free Floating RAS (Rail Adapter System) that has full length quad Mil-Std 1913 rails. This rail system does not use a sleeve like the Mod 0. Optics would be mounted right to the upper receiver or the rails on the Free Floating RAS. The KAC folding back-up sight is used on the rear of the upper receiver rail and a folding front sight is used. The gas block is made at Crane. The Mod 1 uses A.R.M.S. #22 High scope rings that attach right to the rail. The Mk12 Mod 1 upper receiver used during the research of this article was provided by Lamont LeClair, active duty SEAL and owner of Centurion Arms. Monty was able to provide much insight to how the Mk12 is used and what it really brings to the table for SOF units.

The New Improved SOCOM 5.56x45mm Cartridge

In 1999, SOCOM requested that Black Hills work with them jointly to develop the MK12 Special Purpose Rifle (SPR) weapon system. SOCOM was to develop the rifle and Black Hills Ammunition was to develop the ammunition the new rifle would shoot. This rifle was to be accurate out to 600 yards. The load would use the proven Sierra 77-grain open tip match projectile of the AMU. To meet the requirements the cartridge must be “militarized.” This included switching from .223 Rem. to 5.56mm cartridge cases, loading to the increased 5.56mm pressures, crimping and sealing the primers and adding flash retardant to the powder blend. Black Hills Ammunition has developed the first 5.56mm sniper cartridge, the MK262 Mod 0 cartridge adopted in 2002.

During evaluation of the new round, issues came up with reliability when the temperatures dropped and the guns got dirty (external dirt, not ammunition). Issues with short stroking when the rifles were in these conditions without sound suppressors were encountered in the cold with the SPR, which uses a 2 inch shorter barrel than the original 20-inch M16A2 gas system the SPR was built on. Black Hills got right on the problems and through switching to a slower burning powder with a pressure curve tweaked for the 18-inch SPR barrel, the MK262 Mod 1 was born. Later during extremely rigorous function testing at Black Hills, when the weapons were fired at rates greatly exceeding the 12 to 15 round spec rate of fire for the M16/M4 weapon system, it was found that the new propellant was more sensitive to heat from the chambers of hot weapons. This resulted in the increased pressure and increase incidences of failure to extract. Black Hills notified NSWC-Crane and set out to work again to improve the load. By working on a powder blend with higher heat tolerance and improving the brass, these issues were overcome. Another issue that needed to be addressed during the product improvement was Black Hills desire to have Sierra manufacture a cannulure on the 77-grain OTM projectile. Sierra feared this would affect the accuracy of the projectile. Black Hills knew that this round is being used in an autoloading rifle and wanted to avoid the possibility that a rough feed could cause the bullet to push back or telescope back into the case, resulting in a malfunction. Sierra agreed to produce the cannilured version of the projectile. The new and final round was named the MK262 Mod 1 in 2003 and with the correction of the temperature sensitive powder the specification changed but remained the Mod 1.

The Mk262 Mod 1 has gone on to be the most sought out ammunition in the 5.56mm line up for the U.S. military. Primarily used Mk12 SPRs, it has also proven to increase the accuracy and lethality of the 14.5 inch M4 as well as the Mk18 CQB with a 10.5 inch barrel. This author has shot a Mk12 Mod 1 at a silhouette steel target at 850 yards consistently, which is way out of the range of a standard 5.56mm/.223 Rem. caliber rifles and ammunition. The combination of rifle and ammunition has served admirably in both Iraq and Afghanistan. Special operations troops have great confidence and there is an interest in clones of this rifle in the commercial market. For someone looking to build a Mod 1, Centurion Arms offers a complete upper receiver down to the last detail including a Douglas barrel. If one wished for a Mod 0, the upper receivers are built and sold by Precision Reflex, Inc.

Issue box of the highly regarded Mk262 Mod 1 ammunition that was designed with the Mk12 SPR.