Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The SR-556 is a semi-automatic rifle carbine manufactured by U.S. firearms company Sturm, Ruger & Company, Inc.. The rifle was introduced in 2009, marking Ruger''s entry into the popular AR-15 market. It is one of several new AR-15 platform rifles to utilize gas piston operation. The rifle features a patent-pending, "two stage" piston mechanism as opposed to the standard AR-15 direct impingement system. Gas flow is controlled by a four position regulator. Several key parts, such as the gas piston, gas regulator, and bolt/carrier group are chrome plated. The bolt carrier features an integral lug in place of the AR-15''s gas key. The flash hider is similar to Ruger''s AC-556 and Mini-14GB.