[2][3] SPR initially stood for Special Purpose Receiver as it referred to an add-on upper receiver assembly (part of the proposed SOPMOD upgrades),[4][5] but that nomenclature changed to Special Purpose Rifle as the weapon became a stand-alone weapons system.

[6] The weapon was developed by the Naval Surface Warfare Center Crane Division for US military special operations units.

[7] Early experience in influencing the SPR was from American forces deployed in Somalia when they used different optics, ammo, triggers, free float hand guards and rail systems for their rifles.

[8] The SPR concept was originally proposed by Mark Westrom, president of ArmaLite, while working at Rock Island Arsenal in 2000.

[9] 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 shorter than an SR-25.

The Naval Surface Warfare Center, Crane Division expanded on the Recon Rifle.

[7] Initial tests were conducted in October 2000 in order to determine any weaknesses with the first SPR rifles manufactured.

[4] The Mk 12 Mod H was used primarily by Army SOF and Special Mission Units (SMU), and featured a retrofit upper of the Mod 0 with PRI Gen3 round handguards and a shorter RECCE rail to replace the original Swan Sleeve, reducing weight, and either Douglas or Noveske barrels made with the SPR profile in a rifle length gas system.

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.

This increases the accuracy of the weapon by removing vibration and pressure exerted on the barrel by the rest of the gun.

The SPR/A, SPR/B, and MK 12 MOD 1 all use the Knights Armament Company M4 Match Free-Floating Rail Adapter System, KAC part number 99167.

The original SPR used an early PRI flip-up front sight with an elevation dial, which has since been discontinued.

[citation needed] Currently, a Harris swivel model bipod is typically used with the SPR, and is sometimes seen with a KMW Pod-Loc tension adjustment device.

[11] As mentioned above, the bipod is mounted via an ARMS #32 throwlever device attached to the bottom rail of the rifle's forearm.

The first production batches were designated Mk 262 MOD 0 and used a Sierra MatchKing 77-grain (5 g) Hollow Point Boat Tail bullet without a cannelure (crimping groove).

Precision Reflex Inc unveiled a Mk 12 rifle chambered in .308 Winchester in SHOT SHOW 24 in January 2024.