The Ruger AR-556 rifle is a gas impingement driven, box magazine-fed, autoloading rifle. The mechanism utilizes the gas from the firing of the round to drive the bolt carrier rearward and rotate a bolt that locks into the barrel. The AR-556 rifle can be field stripped without the use of tools and breaks down into basic subassemblies. The safety, located above the trigger on the left side of the lower receiver, blocks the trigger and permits the bolt carrier to be cycled with the safety in the on or “SAFE” position. A bolt catch mechanism is provided for convenience in holding the bolt open for loading and inspection. The firing pin is retracted mechanically as the bolt starts to unlock and the rifle will fire only when the bolt is locked.