This weapon system, used by Special Operations Forces units of both the U.S. Army and U.S. Navy, is a heavily modified light sniper/designated marksman variation of the AR-15/M16 line of infantry weapons, and is chambered for NATO standard 5.56 x 45 mm caliber ammunition. The SPR concept was originally proposed by Mark Westrom, currently president of Armalite, while working at Rock Island Arsenal. The program was an outgrowth of the desire by both US Army and Navy special operations forces for a rifle with greater effective range than an M4 Carbine but still shorter in length than a standard issue M16A2/A4. The SPR program appears to have grown out of both the SOPMOD Block II program, and the U.S. Navy SEALs'Recon Rifle' (a 16" flat-topped AR-15/M16 Carbine). The Naval Surface Warfare Center, Crane Division (often referred to as NSWC-Crane or "Crane") essentially expanded on the Recon Rifle, an expansion that some SEALs maintain was a waste of time and resources.
The exact history of the Mk 12 is still in debate, but there appear to be either four or five prime iterations of the weapon, culminating in the most recent Mk 12 Mod 1 version. One progression has four models: SPR Proto 1, SPR Proto 2, Mk 12 Mod 0 and Mk 12 Mod 1. The other progression has five models: SPR, SPR/A, SPR/B, Mk 12 Mod 0, and Mk 12 Mod 1. The specifications in this article follow the second progression.
There is also increasing agreement among observers and small-arms historians that different U.S. military service branches typically deploy different iterations of the SPR. Available evidence, including both U.S. Department of Defense (DOD) photographs and privately-obtained photographs (like the ones shown below), consistently show most US Army SF operators using the Mk 12 Mod 0, while NAVSPECWAR operators and US Army Rangers have been identified as using the Mk 12 Mod 1 version.
The SPR was not well-known to the general public until recently, but it has been featured prominently both in media photos of the Iraq conflict and in interactive video games such as the government-created America's Army: Special Forces, in Tom Clancy's Ghost Recon 2, and the freeware first-person shooterTrue Combat: Elite. Civilian copies of the SPR (commonly called "SPR clones") have also become quite popular among shooters and collectors in the U.S., with several reputable builders of AR-15 style rifles making civilian-legal copies of this accurate rifle.
Upper Receiver: The majority of the SPR upper receivers were initially supplied by Colt, with others being produced by the Canadian firm Diemaco (now Colt Canada). Colt had been outsourcing parts of its production to Diemaco for several years, then purchased Diemaco in February 2005. There is some debate about whether the upper receivers for the later Mk 12 rifles came solely from Armalite, or were a mix of receivers from Armalite and Colt/Diemaco. All of these upper receivers are flat-topped, but have been seen with either the old-style teardrop forward assist and the newer round style.
Lower Receiver: When the SPR program was still just an upper receiver assembly (and not a complete rifle), Crane assembled all of its prototypes using either M16A1 or M4A1 lower receivers. It is unknown whether this pattern continued as the rifle evolved. There is also some issue about whether, when the Navy type-classified the weapon, Precision Reflex Incorporated (PRI) began assembling the rifles themselves. While a number of trigger options were tried in the end, the Knight's Armament Company (KAC) 2-stage trigger was finally decided upon as the standard.
Barrel: An 18-inch (457 mm) threaded-muzzle match-grade free floating stainless steel heavy barrel with a 1:7 (178 mm) rifling twist ratio is standard for the SPR. The barrels are manufactured by Douglas Barrel with a special contour to maximize accuracy and to minimize weight. An OPS Inc. muzzle brake and collar (to align the OPS Inc. 12th Model Suppressor) is installed with the barrel. These barrels were designed to take advantage of the new Mk 262cartridge, which uses a 77-grain (5 g) bullet.
Buttstock: SPRs have been seen with M16A1 or M16A2 fixed buttstocks, telescoping M4 buttstocks, and the Crane Enhanced telescoping buttstock. The rifles are compatible with any type of stock system developed for the AR-15/M-16 weapon system.
Handguards: In all cases a free-floating forearm is used, which does not touch the barrel directly. This increases the accuracy of the weapon by removing vibration and pressure exerted on the barrel by the rest of the gun. The first SPRs used PRI Gen I or Gen II carbon-fiber free-float tubes. The SPR/A, SPR/B, and Mk 12 Mod 1 all use the Knights Armament Company (KAC) M4 Match Free-Floating RAS (Rail Adapter System). The Mk 12 Mod 0 uses PRI Gen III free-float tubes. The Gen I and Gen II Freefloat Forearms are combined with the Atlantic Research Marketing Systems (ARMS) #38 SPR MOD Sleeve, while the Gen III Freefloat Forearm, due to its it larger barrel nut, only works with the ARMS #38 SPR PEQ-2-3.
Sights: The original SPR used an early PRI flip-up front sight with an elevation dial, which has since been discontinued. The Mk 12 Mod 0 uses the current PRI flip-up front sight. The SPR/A, SPR/B, and Mk 12 Mod 1 use the KAC rail foreend flip-up front sight. The SPR and Mk 12 Mod 0 use the ARMS #40 flip up rear sight. The rest of the models use the KAC 600 meter flip up rear.
Optics: Due to the relative modularity of the system, optics (as well as almost everything else) can be mounted according to the operator's wishes. However, SPRs are most often seen with a 3.5–10×40 mm Leupold LR M3 (SPR/A), a 2.5–9×36 mm TS-30 (SPR/B), or a 3–9×36 mm TS-30 A2 (Mk 12 Mod 0/1) Mid Range/Tactical Illuminated Reticle Dayscope (civilian versions are known as the Leupold Mark 4 MR/T 3–9×36). Night vision devices can also be attached. These scopes usually come with flip open dust covers and a honeycomb anti-glare anti-reflection device (ARD). Given Nightforce Optics' recent NAVSPECWAR contract, it is believed that many NAVSPECWAR issued SPRs will use the Nightforce 2.5-10x24 NXS scope 
Mounts: As mentioned before, a long accessory rail, called a SWAN Sleeve (ARMS SPR MOD or ARMS #38 SPR PEQ-2-3), manufactured by ARMS, is installed, running the length of the rifle. The SPR/A and SPR/B both used the KAC M4 Match FF RAS. Two ARMS #22 Throwlever 30 mm steel rings are used to mount the dayscope. The SPR/A, SPR/B, and Mk 12 Mod 1 use ARMS #22 high rings, while due to the increased height from the SWAN Sleeve, the SPR and Mk 12 Mod 0 use ARMS #22 medium rings. An under-the-handguard ARMS #32 Throwlever mount is used to mount the Harris bipod (the ARMS #42 Throwlever mount is used to mount the Versa-Pod); this features a quick release action.
Bipod: Originally Versa-Pods (a cheaper Chinese-made copy of the relatively expensive Parker-Hale swivel bipod) were used, but were taken off the system after the initial SPR. Currently, a Harris swivel model bipod is typically used with the SPR, and is sometimes seen with a KMW Pod-Loc tension adjustment device. As mentioned above, the bipod is mounted via a ARMS #32 throwlever device attached to the bottom rail of the rifle's forearm. The ARMS mount is used on both the Mod 0 and Mod 1.
Suppressor: The The OPS Inc. 12th Model SPR Muzzle Brake Suppressor (MBS) threads directly onto the OPS Inc. muzzle brake and uses the collar to stay centered.
Ammunition: The SPR is not used to fire standard issue 5.56 x 45 mm NATO M855 ball or M856 tracer ammunition, and especially not M193 ball ammunition. Due to the limits in terminal performance and relatively poor accuracy of the 62-grain (4 g) M855 ball, the Mk 262 Open Tip Match (OTM) round was developed as a more accurate round for the SPR, and manufactured by Black Hills Ammunition. The first production batches were designated Mk 262 Mod 0 and used a Sierra Bullets MatchKing 77-grain (5 g) Hollow Point Boat Tail (HPBT) bullet without a cannelure (crimping groove). Black Hills then approached the Nosler bullet manufacturing company, who made a similar 77gr OTM bullet, and Nosler agreed to supply cannelured bullets to Black Hills. The newer load was designated Mk 262 Mod 1. Recently, Sierra agreed to add a minimal crimp to their bullet, and this has since replaced the Nosler bullet in the current versions of Mk 262 Mod 1.
US Army SF operator carrying an OD green-painted Mk 12 Mod 0. The front end of the rifle (with suppressor removed) details the OPS Inc. muzzle brake, the Versa-Pod bipod (folded) and Insight Technologies AN/PEQ-2A infrared laser illuminator mounted on the side of the handguards.
US Navy SEAL takes aim with his desert camouflage-painted SPR. An Insight Technologies AN/PEQ-2A Target Pointer/Illuminator Aiming Light (TIPAL) is mounted on the right of the rifle's handguards. A standard M4 telescoping stock is mounted.