The AUG is an Austrian 5.56 mm assault rifle, designed in the early 1970s by Steyr Mannlicher GmbH & Co KG (formerly Steyr-Daimler-Puch). The AUG (Armee Universal Gewehr - "universal army rifle") was adopted by the Austrian Army as the StG 77 (Sturmgewehr 77) in 1977, where it replaced the aging 7.62 mm StG 58 automatic rifle (a license-built FN FAL). In production since 1978, it is the standard small arm of the Austrian Bundesheer and various police units. It has also been adopted by the armed forces of Argentina, Australia (accepted into service in 1985 and manufactured by Australian Defence Industries in Lithgow, this Austeyr model is also in use by New Zealand), Bolivia, Ecuador (1988), Ireland, Luxembourg, Saudi Arabia, Tunisia (introduced in 1978), Pakistan and since 1988, the U.S. Immigration and Customs Enforcement agency.
The AUG was designed as a family of rifles that could be quickly adapted to a wide variety of roles with the change of the barrel to a desired length and profile, among which are: a compact barrel, carbine barrel, standard rifle-length barrel and a light machine gun barrel. The AUG is a modular, bullpup configuration rifle that employs a high level of synthetic and advanced alloy components.
The primary variant of the rifle designated the AUG A1, consists of six main assemblies: the barrel, receiver with integrated telescopic sight, bolt and carrier, trigger mechanism, stock and magazine. The AUG is a selective fire weapon with a conventional gas piston operated action that fires from a closed bolt. The gas cylinder is offset to the right side of the barrel and works with one of the two guide rods. The rotating bolt features 7 radial locking lugs and is unlocked by means of a pin on the bolt body and a recessed camming guide machined into the bolt carrier. The bolt carrier itself is guided by two guide rods brazed to it and these rods run inside steel bearings in the receiver. The guide rods are hollow and contain the return springs. The bolt also contains a claw extractor that forms the eighth locking lug and a spring-loaded "bump"-type casing ejector. The AUG uses a short-stroke piston system where the right guide rod serves as the action rod, transmitting the rearward motion of the gas-driven piston to the bolt carrier. The left-hand rod provides retracting handle pressure when connected by the forward assist and can also be used to remove fouling in the gas cylinder by utilizing the left-hand guide rod as a reamer. The firearm uses a 3-position gas valve (the first setting, marked with a small dot, is used for normal operation, the second setting illustrated with a large dot – fouled conditions while the third, "GR"-marked closed position is used to launch rifle grenades of the non-bullet trap type), a two-stage trigger (pulling the trigger halfway produces semi-automatic fire, pulling the trigger all the way to the rear produces fully automatic fire) and a safety mechanism (cross-bolt, button type), located immediately above the hand grip. In its “safe” position (white dot) the trigger is mechanically disabled; pressing the safety button to the left exposes a red dot and indicates the weapon is ready to fire. Some versions have an ALO or "automatic lockout", a small projection at the base of the trigger. In the exposed position the ALO stops the trigger being squeezed past the single shot position. If needed, the ALO can be pushed up to permit automatic fire.
The AUG is hammer-fired and the firing mechanism is contained in the rear of the stock, near the butt, covered by a synthetic rubber shoulder plate. The hammer group is made entirely of plastics except for the springs and pins and is contained in an open-topped plastic box which lies between the magazine and the buttplate. During firing the recoiling bolt group travels over the top of it, resetting the hammer. Since the trigger is located some distance away, it transmits its energy through a sear lever which passes by the side of the magazine. The firing pin is operated by a plastic hammer under pressure from a coil spring.
The rifle is fed from translucent, double-column box magazines (molded from a high-strength polymer) with a 30-round capacity and an empty weight of . The light machine gun version of the AUG uses an extended 42-round magazine.
The quick-change barrel used in the AUG is cold hammer-forged by GFM-GmbH of Steyr Austria for increased precision and durability, its bore, chamber and certain components of the gas system are chrome-plated. The standard rifle-length barrel features 6 right-hand grooves and a rifling twist rate of 1 in 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. Three-pronged, open-type flash hiders were used on the , and length barrels, whereas the light machine gun barrel received a closed-type ported muzzle device (combination flash suppressor and compensator) and an integral, lightweight folding bipod. The flash suppressors are screwed to the muzzle and internally threaded to take a blank-firing attachment. Rifles equipped with and barrels are able to launch rifle grenades. pattern barrels produced for military purposes are also equipped with a bayonet lug. The manufacturer offers two other barrel configurations: the first – fitted with a fixed, post foresight (used on the standard rifle version with aperture iron sights) and the second type – equipped with a 40 mm M203 grenade launcher that can be used mounted on the standard length rifle or autonomously – as a stand-alone grenade launcher after attaching a shoulder pad to the end of the 5.56 mm barrel.
The receiver housing is a steel-reinforced aluminum extrusion finished with a baked enamel coating. It holds the 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 with 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 AUG lacks a bolt release button, and the cocking handle must be retracted to release the bolt group after a new magazine has been inserted. Integrated with the receiver casting is a fixed carry handle that contains a 1.5x telescopic sight made by Swarovski Optik. It contains a simple black ring reticle with a basic rangefinder that is designed so that at a tall man-size target will completely fill it, giving the shooter a relatively accurate method of estimating range. The sight cannot be set to a specific range but can be adjusted for windage and elevation for an initial zero and is usually calibrated for 300 m. The rifle also has a back-up iron sight with a rear notch and front blade, cast into the top of the aluminum optical sight housing, used in case of failure or damage to the primary optical sight. The sight is also equipped with a set of three illuminated dots (one on the front blade and two at the rear) for use in low-level lighting conditions. In order to mount a wide range of optics and accessories, a receiver with a NATO-standard Picatinny rail and detachable carry handle was also developed and introduced in 1997.
The rifle’s stock is made from fiberglass-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 the end of the bolt path. The butt is closed by an endplate which is held in place by the rear sling swivel. This swivel is attached to a pin which pushes in across the butt and secures the plate. The rifle is fully ambidextrous. It can be configured for use by left-handed shooters by simply changing the bolt and moving a blanking cap from the left ejection opening to the right.
The AUG uses the 5.56x45mm NATO cartridge and will stabilize both SS109/M855 and M193 bullets.
A semi-automatic version of the rifle known as the AUG P is available to the civilian and law enforcement markets. It features a shorter, barrel and a modified bolt, carrier and trigger assembly that will only allow semi-automatic fire. The rifle also has a slightly different optical sight that features a reticule with a fine dot in the center of the aiming circle, allowing for more precise aiming.
The light machine gun variant can be modified to fire from an open bolt (called the AUG LMG in this configuration). To accomplish this, a modified bolt carrier, striker and trigger mechanism with sear are used.
Based on the AUG, Steyr developed the 9 mm AUG submachine gun that fires the 9x19mm Parabellum pistol cartridge. It is an automatic, blowback-operated model that fires from a closed bolt. Unlike the rifle variants, this SMG has a unique barrel with 6 right-hand grooves at a 250 mm (1:9.8 in) rifling twist rate, ended with a recoil compensator, a slightly different charging handle and a magazine well conversion insert enabling the use of standard 25-round box magazines from the Steyr MPi 81 and TMP submachine guns. A conversion kit used to transform any rifle variant into the submachine gun is also available. It consists of a barrel, bolt, adapter insert and magazine.