The MK-12 special purpose rifle (SPR) was designed by the U.S. Navy to bridge the gap between the M4 assault rifle and the M24 sniper rifle. It is based off of the AR-15 platform making it semi-automatic unlike the M4 and M16 which are fully automatic. It is chambered in 5.56x45mm NATO and is meant to be used with the 77 grain version MK-262. It has an 18" barrel giving it much more range capabilities than the M4 and the 77 grain MK-262 gave it more range capabilities than the M16. The rifle has a maximum effective range of 700 meters or 770 yards while the M4 has 500 meters and the M16 has 550 meters. It was issued with a Leupold MK-4 2.5-8x36mm scope that was specifically calibrated for the 77 grain ammunition on an 18" barrel. Some MK-12 rifles had a standard A2 stock found on M16 rifles and some had collapsible crane stocks found on MK-18 and M4 rifles. It had a free-float hand rail which gave it superior accuracy by having the barrel completely unaffected by the hand rail. It certainly isn't a light weight rifle weighing in at 10 to 12 pounds depending on if you had a suppressor or not.