Steyr AUG

The AUG, referred to as the SUG sometimes, is an Austrian-owned bullpup Assault Rifle designed in the early 1970s.

Overview
During the Peace Walker Incident in 1974, Big Boss and his Militaires Sans Frontieres managed to procure design specs of the SUG after successfully sneaking through the Cloud Forest in Central Heredia undetected, and developed their own versions.

Post-Peace Walker
The Steyr AUG started production in 1978 and later entered into the Austrian military in 1979. It has also found its way into the service of several nations armed forces and governmental organizations in some of these nations. By 1999, Big Boss, by that point the President of Zanzibar Land, had made the Steyr AUG the standardized weapon of his nation, replacing the Enfield L85A1 that acted as their standardized weapon up to 1998, as well as had the military tape their magazines to the sides of their weapon to make reloading easier in battle.

In-game weapons descriptions
"A newcomer to the assault rifle field. Most of its main components are made of plastic, and it features an advanced bullpup design with the action mechanism situated behind the grip. Design-wise, its compact size makes the SUG easy to handle and recoil easier to absorb. However, it is somewhat lacking in power since it uses the same ammo as the M16. The SUG is easy to control due to its low recoil, improving the user's ability to concentrate fire into a target's vital areas and thus boosting the chances of scoring a critical hit."

- SUG weapon description from Metal Gear Solid: Peace Walker