The U.S. Navy's Mk 12 Mod 0/1 Spercial Purpose Rifle (SPR) is a variant of the M16/M4-series rifles in-service with American and allied forces.  U.S. special operations forces have been using this rifle since the onset of OPERATION ENDURING FREEDOM and OPERATION IRAQI FREEDOM.  The rifle has been type classified as a stand alone weapons system rather than a modification of the M16/M4-series from which it is derived.  The M16/M4-series that have been specially modified for combat are called SOPMOD (for special opeations modifications) rifles, but retain their own type designation.  The SPR is significantly different from the SOPMOD rifles to receive its own type designation.  It is used by all the U.S. special operations forces.

The Mk 12 Mod 0 or Mod 1 is specially designed for snipers or designated marksmen.  Mark Weston, now presiden to Armalite, Inc. (a manufacturer of M16/M4-series rifle clones), proposed the Mk 12 modification while he was employed at Rock Island Arsenal, Rock Island, Illinois.  The idea was to prove an accurized rifle that had a longer range than the M4 carbine and a shorter overall length than the M16.  Meanwhile, the Naval Surface Weapons Center, Crane, Indiana, was working on a specialized "Recon Rifle" for the Navy's SEAL teams.  It is unclear on how these two programs evolved into the current Mk 12 Mod 0 and Mod 1 platforms, but there appears to have been four or five different iterations in the design.

Based upon current operational photos and public statements, it appears that U.S. Army Special Forces use the Mk 12 Mod 0 configuration and the U.S. Navy, Marine Corps, and U.S. Army Rangers use the Mk 12 Mod 1 variation.  Civilian copies (called "SPR clones") of the SPR rifle have also become quite popular among shooters and collectors in the U.S., with several builders of AR15-style rifles making civilian-legal copies of this very accurate rifle.

Specifications

• Upper Receiver: The upper receiver blocks themselves were initially supplied by Colt, with some of those being from Diemaco as well (Colt has been outsourcing parts of its production to the Canadian firm for a while now—Colt purchased Diemaco in February 2005, but continues to supply firearms under both names). There is a debate about whether the upper blocks for the Mk 12 rifles, though, came solely from Armalite, or were a mix of receivers from Armalite and Colt/Diemaco. All of these upper receivers were flat top, but have been seen both with the old style tear drop forward assist and the new round style. There is no practical difference between the two forward assists; the difference is only cosmetic.
  
• Lower Receiver: When the SPR program was still in the reciever only stage of development, NSWC Crane assembled all of its prototypes using either M16A1 or M4A1 lowers. It is unknown whether this pattern continued as the program progressed. It is not known if Navy type-classified rifles were assembled by Precision Reflex, Inc. (PRI).  A number of options were tried, and a Knight's Armament Co. (KAC) two-stage trigger desing was standardized.
  
• Barrel: A 18-inch (457 mm) barrel with a threaded-muzzle, match-grade, free-floating stainless steel heavy barrel is standard.  The barrel has a 1 in 7-inch (178 mm) rifling twist ratio. The barrels have a special contour to maximize accuracy and minimize weight and are manufactured by Douglas Barrels.  An OPS Inc. muzzle brake and collar is fitted.  The muzzle brake and collar align the OPS Inc. suppressor is installed on the barrel. The barrels are designed to take advantage of the new Mk 262 cartridge.  The Mk 262 cartridge uses a 77-grain (5-gram) bullet designed specifically for the rifle.
  
• Buttstock: SPRs have been seen with M16A1 buttstocks, M16A2 buttstocks, retractable M4 buttstocks, and the Crane Enhanced Retractable Buttstock (ERB).  SPRs are readily compatible with any type of stock system developed for the AR-15 weapon system.
   
• Handguards: In all cases, a free floating forearm is used. The handguard does not touch the barrel to increase the accuracy of the rilfe by removing harmonic vibrations where various parts contact it.  First SPRs use a PRI carbon fiber free-floating tube that surrounds the barrel.  Later variants and the Mk 12 Mod 1 rifles use the KAC M4 free-floating RAS (rail adapter system).  Mk 12 Mod 0 rifles use an A.R.M.S. No. 38 SPR modification sleeve or No. 38 PEQ-2-3 forearms with larger size barrel nuts.
   
• Sights: Original SPRs used early PRI flip-up front sights with an elevation dial (since discontinued). The Mk 12 Mod 0 uses the current PRI flip-up front sight. Other variants of the rifle and Mk 12 Mod 1 use the KAC rail forearm flip-up front sight. The SPR and Mk 12 Mod 0 use the A.R.M.S. No. 40 flip-up rear sight. The rest of the models use the KAC 600 meter flip up rear.
  
• Optics: Due to the modularity of the system, optics (as well as almost everything else) can be arranged according to the operator's wishes.  The Mk 12 system is most often seen with a 3.5�?0×40 mm Leopold LR M3 or a Leopold 2.5�?×36 mm TS-30 or a Leopold 3�?×36 mm TS-30 A2.  Civilian versions are known as the Leopold Mark 4 MR/T 3�?×36). Night vision devices can also be attached. These scopes usually come with Butler Creek flip-open dust covers and a honeycomb anti-glare anti-reflection device (ARD) called Kill-Flash.  New production Mk 12 Mod 1 rifles will use the  Nightforce 2.5-10x24 NXS scope.  
  
• Mounts: An accessory rail, called a SWAN Sleeve (A.R.M.S. SPR MOD or A.R.M.S. No. 38 SPR PEQ-2-3) is installed.  The sleeve runs the length of the rifle. Some rifles use the KAC M4 Match FF RAS. Two standard medium or high A.R.M.S. No. 22 throw lever 30 mm steel rings (or one standard No. 22 ring and a No. 22 Tactical Ring Rail [TRR]) are used to mount the day scope. An under-the-handguard A.R.M.S. No. 32 throw lever mount is used to attach the Harris bipod (an A.R.M.S. No. 42 throw lever mount is used to attach a Versa-Pod bipod with aquick-release lever).
  
• Bipod: On the first rifles, a Chinese copy of the expensive British Parker-Hale bipods were fitted.  (These bipods are commercially known as the Versa-Pod.)   Current rifles use a Harris swivel model bipod and it is sometimes fitted with a KMW Pod-Loc tension adjustment device.
  
• Suppressor: The suppressor threads directly onto the OPS Inc. muzzle brake and uses the installed barrel collar to center the device. The suppressor is an OPS Inc. 12th Model SPR Muzzle Brake Suppressor (MBS).
  
• Ammunition: The SPR is designed for the Mk 262 77-grain (5-gram) open tip match (OTM) developed by Black Hills Ammunition.  The Mk 262 Mod 0 used Sierra Match King 77-grain (5-gram) hollow point boattail (HPBT) bullets.  The Mk 262 Mod 1 cartridge developed later used a similar projectile made by Nosler.  The Mod 1 is now standardized.  Both Nosler and Sierra are providing projectiles under the Mod 1 standard.  The M855 ball, M856, and M193 cartridges are not issued for use in this rifle.   The cartridges have lighter projectiles that neutralize the accuracy built into the Mk 12 weapons platform.

Above:  A Mk 12 Mod 0 prototype rifle with a standard butt stock fitted.  This rifle has a Leopold scope with the illuminated reticle option and A.R.M.S. No. 22 medium height rings; the rear is a standard throw lever ring and the front is a No. 22 Tactical Ring Rail or TRR (see bottom photo).  The platform above the scope tube allows mounting of other targeting devices.  The PRI free-floating tube is fitted along with the SWAN Sleeve RAS along with folding front and rear sights.  An amidexterous saftey selector is fitted along with an ergonomic (human-engineered) pistol grip.  This prototype rifle uses the Versa-Pod bipod.  Note the suppressor attached to the barrel.  (Photo: Department of Defense)

Below:  A Mk 12 Mod 1 rifle shows the differences between rifles.  This rifle has the A.R.M.S. No. 22 high height throw lever rings.  Note the different free-floating KAC sleeve as compared to the PRI, the relocated front and rear sights, Harris bipod, and the Leopold 2.5 to 9 power day scope with A.R.M.S. No. 22 rings, and difference in size of the barrel nuts between the Mod 0 and Mod 1 rifles.  (Photo: Department of Defense)

Above: An Army Special Forces sniper explains the operation of his Mk 12 Mod 0 rifle to his Filippino counterpart.  This rifle is equipped with the Versa-Pod bipod.  (Photo U.S. Army)

Below: Another Special Forces sniper with a Mk 12 Mod 0 SPR built on an M4 carbine receiver.  Note that the suppressor has been removed from this rifle to show the OPS Inc. muzzle brake and collar used to center the suppressor on the barrel.  The bulky item on the right side of the forearm near the front sight is an AN/PEQ-2A infrared illuminator and sight.  The Versa-Pod bipod is folded backwards in this shot. The object just ahead of the elevation knob on the scope is an A.R.M.S. No. 22 Tactical Ring Rail -- a modification of the standard No. 22 throw lever ring, the extension allows for mounting of other tactical sights and targeting devices.  (Photo: U.S. Army)