Staff Writer (Updated: 4/15/2016):
The Mk 12 was born out of a joint US Army/Navy requirement for a compact, semi-automatic capable sniper rifle system. Multiple prototypes were drawn up and these were then heavily tested resulting in the finalized "Mk 12" form. Like the AR-15 and M16 before it, the Mk 12 was chambered for a specially-developed version of the 5.56x45mm NATO cartridge firing from a semi-automatic, gas-operated rotating bolt action. The weapon feeds from a 20- or 30-round detachable box magazine and retains much of the AR-15/M16 form and functionality. Effective range is listed out to 600 yards with a muzzle velocity reported at 3,050 feet per second. The pistol grip is ergonomically grooved for a firm hand hold while the trigger is set within an integrated guard. The magazine feeds just ahead of the trigger in typical M16-style fashion. The forend is covered over in a perforated, tubular handguard to which a rail system is affixed along the front bottom face - this allows use of a collapsible bipod for steady firing. The upper portion of the receiver and the forend also sport a rail system for the use of accessories and optics as needed. The shoulder stock extends from a collapsible tubular frame and helps the operator achieve a four-point firing platform (when using both hands, a bipod and the shoulder stock) for maximum accuracy at distance. The barrel protrudes only a short distance from the forend and is capped by a single-baffled muzzle brake. The barrel measures in at 18 inches in length while overall length of the SPR is 37.5 inches. A fully-loaded system weighs only 10lbs, slightly more than the original M16. In comparison, the M16 measures in at 39.5 inches long with a 20-inch barrel length.
It should be noted that there are several iterations of the Mk 12 system currently in circulation and this is expected when detailing special forces weaponry. To date there are two distinct identified forms known as the "Mk 12 Mod 0" and the "Mk 12 Mod 1". The formal "long designaton" designation for the rifle system is "United States Navy Mark 12 Mod0/1 Special Purpose Rifle".