Staff Writer (Updated: 8/9/2016):
The AUG was designed around the "bullpup" configuration concept in which all of the major internal working components of the action - and its corresponding magazine feed/ejection system - were concentrated to the rear of the pistol grip. While the style is notably gaining in popularity today, the bullpup configuration - when used in a frontline service rifle - was something of a drastic departure for the time. The rear-set placement of the internal workings allowed engineers to feature a full-length barrel within a more compact overall form. The heavier stock, when properly positioned against the body, allowed for a firmer three-point hold of the weapon, making its use in confined spaces more acceptable than traditional long guns. Accuracy at shorter ranges was improved and an operator could raise his weapon (in response) quicker than a traditional infantryman with a conventionally-arranged long gun could. Considering the ranges at which infantryman often engaged their targets - particularly in urban settings - this made the bullpup approach a rather sound one.
The Steyr AUG took on a most futuristic appearance for its time, showcasing well-contoured lines, minimal external detailing and proven internal functionality. The receiver was a large plastic (fiberglass-strengthened polyamide 66) assembly covering the various metal (aluminum and steel) parts within. The receiver also made up the fixed buttstock and sported the magazine feed as well as the ejection port - all aft of the pistol grip. The pistol grip itself was integrated cleanly into the design and ergonomically engineered for a firm hand hold. The grip was protected by a slim hand guard while the large curved trigger could clearly be identified. A safety was within easy reach of the firing hand. The forward region of the weapon contained the all-important barrel which protruded a distance ahead. A folding vertical foregrip was set beneath the barrel at its base and this grip was used to change out the "quick-change" barrel as needed. Over the top of the weapon was a two-point mounting system for the standard-issue optics (a 1.5x Swarovski scope) that doubled as a carrying handle. This installation could be removed and replaced by a standard Picatinny mounting rail (by way of an alternative receiver casting) for custom optics and accessories in follow-up production models. The steel barrel was capped by a slotted or three-pronged flash hider. In all, the AUG could be field-stripped into just six key components. Ambidextrous operation was made possible by covering the right ejection port and revealing the left side port with a left-handed bolt also replaced the standard right-hand bolt. A bayonet mounting and underslung grenade launcher (M203 40mm) were optional and broadened the tactical usefulness of the AUG family in-the-field. The barrel (16- and 20-inch flavors) was also designed to fire rifle grenades.
One of the distinct features of the AUG action was its "two-stage" trigger system. There was no conventional fire selector per se - as common to other automatic firearms incorporating modes beyond that of semi-automatic - as the trigger was pressure-guided to respond through single-shot firing and full-automatic firing. The first pressure level revealed the basic single-shot fire mode while additional pressure would bring the weapon into full-automatic fire mode.
The AUG is categorized as a gas-operated weapon featuring an internal rotating bolt function. The gas is tapped at the barrel in the traditional sense but the gas cylinder itself is actually offset to the side of the barrel as opposed to directly over or under it (as in the AK47 or M16 respectively). As such, the cylinder doubles as a bolt guide rod during the firing operation. The AUG family is primarily chambered for the 5.56x45mm NATO cartridge (SS109 NATO or M193 bullet) and fed from a standard 30-round curved detachable box magazine that is transparent, allowing the operator to visibly see his current ammunition supply. There is also a 42-round box magazine and support for a Beta C-Mag ammunition drum of 100 rounds. The submachine gun (Para) form utilizes a thinner magazine for its 9mm cartridges of 25- or 32-round counts.
The Steyr AUG was really one of the first "true" modular weapon systems of note, designed around a standard receiver unit which could be adapted for other combat roles beyond that of assault rifle with little modification to the core weapon. Essentially, a change of barrel could create a carbine form, a sub-carbine form and a light machine gun version as needed. A submachine gun form was also possible, though with a bit more internal modification involved. All barrels were steel in their manufacture and inserted by way of an interrupted thread. The standard assault rifle utilizes a 31 inch barrel while the carbine and sub-carbine versions make use of a 27- and 25-inch barrel length respectively. The light machine gun (HBAR) version utilizes a 35 inch barrel assembly and operates with a "heavy barrel" (hence the designation) and a bipod for stabilization in the sustained fire role. The AUG Para submachine gun fires from a 16.5 inch length barrel.
The initial AUG production model became the AUG A1 of 1977. This was followed by the AUG A2 which sported a new cocking handle and support for the MIL-STD-1913 accessories/optics rail. The AUG A3 incorporated the changes of the A2 but brought about an external bolt release feature. The AUG M203 designation was nothing more than an AUG system with support for the M203 40mm breech-loading single-shot grenade launcher in an under-barrel placement. The AUG NATO was developed to support NATO STANAG magazines and came in a right-handed version only.
The AUG A3 SF "Special Forces" (also AUG A2 "Commando") model was a specially-designed compact special forces version. The AUG Para of 1988 was the submachine gun form chambered for the 9x19mm Parabellum cartridge and internally reworked to fire the smaller cartridge through a blowback system of operation. The AUG A3 Para XS was similar in form and function though with a 13 inch barrel and support for Picatinny rail accessories and based on the AUG A3 rifle.
The various "machine gun" classified AUGs were the squad automatic weapon version as the AUG LSW (Light Support Weapon). The AUG HBAR (Heavy Barreled Automatic Rifle) was, therefore, the heavy-barreled version of this weapon which was, itself, essentially the AUG rifle modified for the sustained fire role with its heavy barrel and bipod. The AUG HBAR gave birth to the AUG LMG (Light Machine Gun) which incorporated a 4x scope and open bolt mechanism. Rail support (ala the AUG P Special Receiver) began the AUG LMG-T line. A "designated marksman" version became the AUG HBAR-T with its 6x42 standard optics.
The AUG Z was a civilian-minded semi-automatic only model as was the AUG SA, the latter intended for the American gun market. The USR was an AUG A2 developed to meet the requirements of the US Federal Assault Weapons Ban. The AUG P was a semi-automatic only version of the AUG A1 intended for civilian and police forces sale, incorporating a 16" barrel assembly. The AUG P "Special Receiver" was similar in scope though bringing with it the MIL-STD-1913 rail support.
The AUG family has gone on to see extensive use the world over. Argentina selected the AUG to replace their outgoing American M16A2 rifles. Australian took on the AUG in number beginning in 1989 as the standard issue assault rifle of the Army with local production through Thales Australia. Austria continues use of the AUG family to this day. Other operators include Bolivia, Bulgaria, Cameroon, Croatia, Djibouti, Ecuador, the Falklands Defense Force, Gambia, Indonesia, Ireland, Italy, Luxembourg, Malaysia (production under license), Morocco, Netherlands, New Zealand, Oman, Pakistan, Papua New Guinea, Philippines, Poland, Saudi Arabia, Serbia, Taiwan, Tunisia, the United States (US Immigration and Customs) and Uruguay.
The AUG is known in Australian Army service as the "F88" and comes in various - though inherently similar - forms to suit battlefield requirements. Several US-based companies produce copies of the AUG in various guises for the civilian market.