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Recent(ish) Mk12 article.


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DIdn't catch this because I let my subscription lapse...   FireArms News did an article on the Mk12 a year ago.  Good writeup on the Mk12.  Mainly, the Mod 0.


Best of the AR-15s? Mk12 Mod 0 SPR

The 5.56mm Mk12 Mod 0/1 series turned the M16 into a compact sniper rifle which was wielded with dramatic results by US Special Operations!

Best of the AR-15s? Mk12 Mod 0 SPR

The 5.56mm Mk12 Mod 0 SPR was designed to provide both precision and support fire for US Special Forces. It was successfully fielded in both Afghanistan and Iraq.

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.

MK12-Sniper-Rifle The Black Rifle has come a long way in the last 40 years. The Mk12 series were built using old M16A1 lower receivers.

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.

MK12-Sniper-Rifle The SPR concept over time until it eventually became the Mk12 Mod 0. It sported a lot of very advanced features for the time, including a dedicated optic and load.

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

MK12-Sniper-Rifle The Mk12 Mod 0 is fitted with a muzzlebrake and folding PRI front sight. Note how the weapon is designed to accept a sound suppressor.

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.

MK12-Sniper-Rifle A PRI free-floating handguard is mounted to enhance accuracy. A one-piece A.R.M.S. rail connects it to the receiver.

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.

MK12-Sniper-Rifle Mounted in A.R.M.S. rings is a Leupold 3-9x36mm MR/T Tactical scope with M3 turrets.

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.

MK12-Sniper-Rifle This 5-shot 100-yard group was shot off the bench using Black Hills Ammunition’s 77-grain OTM Match load. It measures .6 inch.

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.

In Combat

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

Operation: Direct-gas
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


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8 minutes ago, blue109 said:



Funny you bring that pic up, brother - I'm modifying the Mod 0.  Keeping the B5 stock on it, but changing the recoil system to the VLTOR A5, instead of just "AR15 Carbine parts."  Running the big A5 buffer in it, the A5H4, at 6.83oz.  The extension is sitting in the PO Box, waiting for my to pick it up in the morning.  Buffer has been here forever.  Can't wait to go shoot it - again.

Edited by 98Z5V
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2 hours ago, Cunuckgaucho said:

Somewhere at a top secret test location...


That's RIGHT!  Geared up for some High Angle training, right there!  Me and my Mk12 Ninja Brother @blue109 right there :hail:  - because many others are just SKEERED about the Mk12s.  :banana:

@Matt.Cross has a Mk12 Grendel, because I made him build it.  And a Mk12 ARC.  Same reason.  I guess he's in on the Mk12 crowd.  Me and Mikey, though - OG.  To the bone. 

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On 5/26/2022 at 2:35 AM, 98Z5V said:

@Matt.Cross has a Mk12 Grendel, because I made him build it.  And a Mk12 ARC.  Same reason.  I guess he's in on the Mk12 crowd.  Me and Mikey, though - OG.  To the bone. 

I love the Mk12 platform! I'm definitely not in the purist crowd about the Mk12, but I did contemplate a standard 5.56-based build or two and you talked me into using ballistically superior rounds. I'm glad you did, I couldn't be happier with the way my Mod 1s perform.

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On 5/26/2022 at 2:35 AM, 98Z5V said:


@Matt.Cross has a Mk12 Grendel, because I made him build it.  And a Mk12 ARC.  Same reason.  I guess he's in on the Mk12 crowd.  Me and Mikey, though - OG.  To the bone. 

Excuse me sir…. I believe i fit in somewhere here…..


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5 hours ago, edgecrusher said:

Excuse me sir…. I believe i fit in somewhere here…..


HERE, HERE!!!   I stand corrected!!!   :hail:  You Sir, are INDEED in the OG Crowd!  :thumbup:

I think I forgot about that gun, because you've never brought it out here to a shoot.  Brought the LaRue, but no Mk12...   :hornet::laffs:

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2 hours ago, ApexJunky said:

I’m not against revamping mine with a betterer caliber 🤔



Grendel Mk12 is amazing.  6 ARC Mk12 is a teeny bit better than Grendel, but not worth converting over if you already have a Grendel.  Mk12 is just the most efficient use of the AR platform, and Grendel is the most efficient cartridge devised for a small-frame AR.  Then, along came ARC...   :laffs:

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Found another good article from Small Arms Defense Journal (love those guys). 


The Mk12 SPR (Special Purpose Rifle)

The Mk12 SPR (Special Purpose Rifle)

Christopher R. Bartocci / 8 January, 2016 / Comments Off on The Mk12 SPR (Special Purpose Rifle) / Features, Reviews, Search By Issue, V7N5, Volume 7

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.

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.

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.

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.

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.

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

The Mk 12, Mod 0 is carried by the U.S. Army Special Forces. 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.

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 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.http://sadefensejournal.com/wp/wp-content/uploads/2015/11/01082016-001-09.jpg

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.

by Christopher R. Bartocci on 8 January, 2016.
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