its bore, chamber and certain components of the gas system are chrome-plated (currently nitrided on US market rifles). The standard rifle-length barrel features 6 right-hand grooves and a rifling twist rate of 228 mm (1:9 in). An external sleeve is shrunk on to the barrel and carries the gas port and cylinder, gas valve and forward grip hinge jaw. There is a short cylinder which contains a piston and its associated return spring. The barrel locks into a steel insert inside the receiver through a system of eight lugs arranged around the chamber end and is equipped with a folding, vertical grip that helps to pivot and withdraw the barrel during barrel changes. The most compact of the barrels has a fixed vertical grip. The receiver housing is a steel-reinforced aluminium extrusion finished with a baked enamel coating. It holds the steel bearings for the barrel lugs and the guide rods. The non-reciprocating plastic cocking handle works in a slot on the left side of the receiver and is connected to the bolt carrier's left guide rod. The cocking handle has a forward assist feature—alternatively called a "silent cocking device"—used for pushing the bolt shut without recocking the rifle. A bolt hold-open device locks the bolt carrier assembly back after the last round has been fired. The newer AUG A3s possess a bolt release button, prior to this development all AUGs and the USR required the cocking handle being retracted to release the bolt group after a new magazine has been inserted. Older versions of the AUG can be upgraded to use the newer A3 stock and hammer pack. The rifle's stock is made from fibreglass-reinforced polyamide 66. At the forward end is the pistol grip with an enlarged forward trigger guard completely enclosing the firing hand that allows the rifle to be operated with winter gloves. The trigger is hung permanently on the pistol grip, together with its two operating rods which run in guides past the magazine housing. Behind that is the locking catch for the stock group. Pressing this to the right will separate the receiver and stock. The magazine catch is behind the housing, on the underside of the stock. Above the housing are the two ejector openings, one of which is always covered by a removable strip of plastic. The rear of the stock forms the actual shoulder rest which contains the hammer unit and