The Ruger SR-556 was Ruger's first AR-platform rifle, employing a two-stage piston system licensed from the Sullivan design. Marketed as a cleaner-running, heat-reducing alternative to direct impingement with a chrome-lined barrel and Troy rail system. Type: Semi-Auto Rifle Action: Piston-Operated Caliber: 5.56x45mm Capacity: 30 Year introduced: 2009, USA Weight: 7.9 oz Overall length: 35.5" Status: Discontinued Intended use: Range, Patrol