The United States Navy Mark 12 Mod 0/1 Special Purpose Rifle (SPR) is a rifle that has been in service with the United States special forces in Operation Enduring Freedom and Operation Iraqi Freedom. SPR initially stood for Special Purpose Receiver, but that nomenclature seems to have disappeared as the weapon became a stand alone weapon system, and not just an add on upper as part of proposed SOPMOD upgrades. SPR now means Special Purpose Rifle. It was eventually type classified by the Navy as the Mk 12, though the Army also uses this designation. This weapon system, used by Special Forces units of both the US Army and US Navy, is a heavily modified deviation from the familiar AR15/M16 line of infantry weapons, and thus is chambered for NATO standard 5.56 x 45 mm caliber ammunition. It serves a light sniper/designated marksman role. The SPR was originally proposed by Mark Westrom, currently president of Armalite, while at the Rock Island Arsenal. The SPR program was an outgrowth of the desire by both US Army and Navy special forces for a rifle with greater effective range than an M4 type 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'