Rifle of the world 3 : Mark 12 Mod 0/1 Special Purpose Rifle (SPR) b

Specifications
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 or 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 262 cartridge, 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.