The STG-556 Sporting Model rifle is a gas operated semi-automatic bull pup rifle featuring a quick interchangeable barrel available in various lengths. This innovative design features a Forward Assist and Last Round Bolt Hold Open Release; something never before seen in a rifle of this distinction. This state-of-the-art design and platform has been totally re-engineered to bring you absolute function and reliability... Making a 30 year battle proven design bar none above all others. Each model comes standard with a 9" picatinny rail.