The Mark 12 Mod 0/1 Special Purpose Rifle (initially Special Purpose Receiver) is an American designated marksman rifle designed by Naval Surface Warfare Center Crane Division (NSWC Crane) from 1998 to 1999 and manufactured from 2000 to about 2011. Initially conceived and designated as a specialized drop-in receiver, its designation was changed when the Mk 12 became a standalone weapons system entirely. The Mk 12 was most notably used by SOCOM units and US Marine Corps units from 2002 to 2017 before it was replaced by the FN SCAR.

History

The initial idea for the Mk 12 came from Mark Westrom, current president of ArmaLite; his original idea envisioned this rifle to feature 18, 20 or 22-inch barrels.^[1] In addition to the above, the weapon was initially designed as an upper receiver which could adapt to the M4s and M16s in use at the time; it would be used as a light marksman rifle most of the time, but if required, it could be converted to be used in a light machine gun role. There was no rifle at the time which could achieve this so SOCOM would have to build it themselves; however, the project went dormant for unknown reasons.^[2]

The project was soon revived as an initiative by the 5th Special Forces Group, envisioning it as a highly-specialized top receiver which is inserted onto the lower receiver of an AR-15-type rifle; this was intended to provide special operations units with a light support capability which were in no position to receive aircraft support or artillery.^[2]

Starting in 1998, the SOPMOD Programs Office at the Naval Surface Warfare Center Crane Division went around drafting the requirements for the Special Purpose Receiver and tested and solicited this concept. Initial prototypes were then constructed by the 5th Special Forces Group in collaboration with the U.S. Army Marksmanship Unit (USAMU) and tested with handloaded match-grade ammunition. SOPMOD funded several of the prototypes. Ultimately, the project was solidified as the requirements were realistic and achievable and the SPR was ultimately validated as part of SOPMOD requirements in October 1999.^[2]

Initial production SPRs were manufactured by NSWC Crane, although later production rifles used Colt, Colt Canada, ArmaLite or Precision Reflex receivers; the first 100 Limited User Test rifles were instructed to be produced in large quantities for field tests with SOCOM operators by Summer 2001. The rifles first saw use in Operation Enduring Freedom around 2001, with most of the initial production rifles sent for use in Afghanistan for field testing. It was here where the SPRs showed what they were capable of, as they were credited with an extremely high percentage of enemies killed with precision rifles. It was also here where the SPR was ultimately finalized by SOCOM as the Mk 12; however, the SPR acronym changed from Special Purpose Receiver to Special Purpose Rifle as the system morphed from being simply a highly modified and specialized receiver for these purposes into an entirely new weapons system.^[2]

Different branches of the US military are said to deploy different iterations of the Mk 12; according to various photographs, whether they be officially issued by the Department of Defense or taken by private photographers, US special forces units are stated to use the Mod 0 version of the Mk 12,^[3] while NAVSPECWAR and U.S. Army Rangers appear to use the Mod 1 iteration.^[4] Mk 12s were also noted to have been used during Operation Red Wings, as noted by retired Navy SEAL Marcus Luttrell.^[5] The rifle was extensively used in Iraq and Afghanistan during the closing stages of the war.^[6][7]

Come 2011, USSOCOM revealed plans to replace their Mk 12s with the FN SCAR SSR by 2017;^[8] the M27 Infantry Automatic Rifle was also highlighted as a potential replacement for the Mk 12.^[9] The weapon has since been replaced in usage by the SCAR SSR.

Centurion Arms manufactures their own version of the Mk 12 as well.^[10]

Design Details

The Mk 12, in essence, is a highly modified derivative of the AR-15 rifle platform. Of note is that the weapons are built to SOCOM specifications and then once shipped over to their respective units the units modify the weapons to their own specifications.^[2]

Most early Mk 12 receivers were initially supplied by Colt and by Diemaco, now Colt Canada; Colt had actually been outsourcing parts from Diemaco for a time before ultimately purchasing the company themselves.^[2] Later Mk 12 receivers seem to come from a variety of sources, although it is not clear as to whether they came solely from ArmaLite or from ArmaLite in addition to both Colt and Colt Canada; all receivers are noted to be flat-topped but feature varying styles of forward assists.^[1] Precision Reflex Inc. (PRI) may have manufactured some of the rifles themselves.

In the project's early days, the Mk 12 prototypes used M16A1 or M4A1 lower receivers with full-auto trigger groups as it was felt that those provided a more consistent pull compared to the three-round burst trigger groups, which were more common. As the rifle evolved, it is unclear if this pattern continued. Ultimately, the trigger group used for the production rifles was a KAC two-stage select fire trigger.^[1] Later versions of the rifle seem to use an Accuracy Speaks single-stage trigger, which is what appears to be used in the last production rifles. The old M16A1 and M4A1 receivers used on the prototypes were subsequently sent back to NSWC Crane for destruction due to their obsolescence, and that more was required to turn the rifles into capable marksman weapons.^[2]

The weapons use 18 in (46 cm) match-grade stainless steel barrels with threaded muzzles with a 1:7 twist ratio.^[7] The initial 150 receivers used barrels manufactured Douglas Barrels, Krieger Barrels Inc. and Schneider Rifle Barrels in an equal 50-50-50 split; after various performance tests the Douglas barrels were used for the full-production rifles. At the end of the barrels sits an Ops, Inc. muzzle brake, which is threaded for an Ops, Inc. suppressor.^[2] These suppressors are no longer manufactured, but an equivalent is manufactured by Allen Engineering as the AEM-5; Ron Allen, who designed the AEM-5, also worked on the Ops, Inc. suppressors for the Mk 12.^[11]

The Mk 12s are shipped with M16A1 or M4A1 fixed buttstocks. However, once the rifles are shipped out to their respective units they were often modified; as such, they can be also found with non-standard M4 telescoping buttstocks or Crane telescoping buttstocks.^[2]

All rifles feature a free-floating forearm to help increase the accuracy of the rifles, with the first generation rifles using carbon fiber free float tubes manufactured by Precision Reflex, while later rifles use a free-floating adapter rail system manufactured by KAC or PRI Gen III free-float tubes.^[2]

The initial SPR prototypes used a now-discontinued PRI flip-up front sight with an elevation dial; this was carried over to the Mk 12 Mod 0. Later Mk 12s, particularly the Mod 1, use PRI Rail Mounted Flip Up Front Sights. The charging handle of the weapon is a PRI Gas Buster charging handle, designed to eliminate any gas from leaking out the upper end of the receiver. Due to the system's modularity, various optics can be fitted, with Leupold optics being common choices. An Atlantic Research Marketing Systems (ARMS) Swan #38 sleeve rail, also known as the Swan Sleeve, was fitted to the rifle and runs along its length. ARMS #22 throwlever mounts are used to mount daytime optics.^[2]

The rifles were initially fitted with swiveling Parker Hale bipods;^[12] these were later replaced with removable Harris or Versa-Pod bipods, mounted on an ARMS #22M throw lever.^[2]

Ammunition

The rifles use a highly-specialized version of the 5.56×45mm NATO named the Mk 262 Open Tip Match, developed specifically for the Mk 12. The round is noted to greatly increase the accuracy of its host rifle, as well as other rifles which use it.^[13] It is said to be highly sought after by the U.S. military.^[2] The ammunition is manufactured by Black Hills Ammunition.^[14]

Variants

Mk 12 Mod 0

National Stock Number 1005-01-504-3275. Version used by the U.S. Army Rangers. Standard configuration consists of an ARMS #38 Swan Sleeve, PRI Gen III free-float forearm, PRI gas block with folding front sight, ARMS Swan #40 standalone flip-up rear sight and ARMS #22 throw levers for optics. Differs from Mod 1 in terms of handguard assembly and backup sights.^[2]

Some twelve rifles were later modified by PRI in 2007 and fitted with new barrels, gas blocks and bolts, new PRI rail-mounted front sights, new forearms, new stocks, new triggers and new charging handles.^[2]

Mk 12 Mod 1

National Stock Number 1005-01-504-3276. Version used by U.S. Army Rangers, U.S. Navy SEALS and the U.S. Air Force Special Tactical Teams. Standard configuration consists of a KAC free-floating RAS system with quad full-length Picatinny rails, KAC folding rear sight, PRI rail-mounted flip-up front sight, NSWC Crane-manufactured gas block and and ARMS #22 throw levers for optics. Differs from Mod 0 in terms of handguard assembly and backup sights.^[2]

Gallery

References