The Mk 18 Close Quarters Battle Receiver is the Navy SEAL standard CQB rifle, featuring a 10.3-inch barrel that maximizes maneuverability in ship and building clearing. Uses the same M4A1 lower receiver with an extremely compact upper. Type: Semi-Auto Rifle Action: Gas-Operated Caliber: 5.56x45mm Capacity: 30 Year introduced: 2004, USA Weight: 5.9 oz Overall length: 28" Status: Active Military use: Yes Intended use: Military, Special Operations