The MK 12 rifle was one of the beginning stages of an “enhanced carbine” concept. The original designation as SPR was intended to mean Special Purpose Receiver but the system was eventually type classified by the Navy as a stand alone rifle and designated MK12 Mod 0/1. The concept started with the idea of having a sniper/designated marksman upper receiver for a special operators standard M4 lower receiver but during initial testing and limited fielding Naval Surface Warfare Center, Crane Division delivered the upper receiver with there own lower so the testing units would not have to deplete the units operational weapons to support the testing. The lowers supplied were mostly M-16A1 lowers with Knights Armament match triggers installed and fixed butt stocks.

Barrel- An 18-inch, threaded muzzle, match-grade, free floating, stainless steel, heavy barrel with a 1:7 rifling twist is standard for the SPR. The barrels are manufactured 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 bullet.

Handguards- 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 Mk 12 Mod 1 uses the Knights Armament Company (KAC) M4 Match Free-Floating RAS (Rail Adapter System).

Sights- Mk 12 Mod 1 use the KAC rail foreend flip-up front sight and the KAC 600 meter flip up rear.

Optics- Current MK12 rifles are being fielded with Night force 2.5x10 scopes and high rings.

Mounts-. 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.

Bipod- An under-the-handguard ARMS #32 Throwlever mount is used to mount the Harris bipod this features a quick release action. a Harris swivel model bipod is typically used with the MK12, 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.

Suppressor- 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 M855 ball, the Mk 262 Open Tip Match (OTM) round was developed as a more accurate round for the SPR, and is manufactured by Black Hills Ammunition.