The United States Navy Mark 12 Special Purpose Rifle is one badass gun. It remains on my list of AR variants that I have to own someday. Information according to Wikipedia:
The SPR, used by Special Operations Forces 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 5.56x45mm 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 M16 carbine). The Naval Surface Warfare Center, Crane Division expanded on the Recon Rifle.
BCM sells complete Mk 12 SPRs as well as all the parts to build your own.
BCM also offers complete upper receiver groups to build the Mk 12 Mod 0 and Mk 12 Mod 1 SPR, or custom variations thereof.
In terms of optics used, there is a lot of variation:
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. Night vision devices can also be attached. These scopes usually come with flip open dust covers and a honeycomb anti-glare anti-reflection device. Given Nightforce Optics’ NAVSPECWAR contract, it is believed that many NAVSPECWAR issued SPRs will use the Nightforce 2.5-10×24 NXS scope
Since the weapon system is modular, there will always be small variation among all Mk 12 SPRs based on operator preference and availability of parts (though in terms of stocks, most used M16A2 fixed or Crane enhanced telescoping [LMT SOPMOD] buttstocks), and receiver variation based on supplier (both upper and lower). Handguards, however, are more straightforward. The Mk 12 Mod 0 uses a PRI Gen III modular 12″ carbon fiber free float handguard. The Mk 12 Mod 1 uses a Knight’s Armament Company (KAC) RAS 12″ free float handguard. Similarly, all Mk 12 SPRs used Douglas 18″ match-grade stainless steel heavy barrels with a 1 in 7″ twist rate and special contour. All Mk 12 SPRs also feature the KAC 2-stage trigger and OPS Inc. muzzle brake. As far as optics mounts:
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.
The gun was designed to fire the Mk 262 cartridge, utilizing 77-grain bullets.
Iron sight selection depended on the Mk 12 SPR variant:
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.
Tactical Arms dedicated an entire episode to the Mk 12 SPR:
More photos available here.