The assault rifle is made of fiberglass reinforced olive-green colored polymer. Because of this, the gun is extremely lightweight, weighing only around 7 pounds. The gun has a scope with telescopic, all purpose practical donut sights that don't require power to operate. The gun holds a clear double column box magazine made of a high-strength translucent polymer that holds 30 5.56x45mm NATO rounds so you can see how many you have left. The gun is capable of firing 680 to 750 rounds per minute and because of this, features an interchangeable barrel and replacement barrels of varying lengths. The gun is bullpup with a rotating closed bolt, which means that the magazine is located on the butt of the gun behind the trigger. It also has a collapsible bipod mount so you can prop the gun up and shoot one handed, if necessary.
Karl used this as his weapon of choice when chasing John McClane. After being informed that McClane killed his brother Tony, Karl flew into a savage rage, throws the office desk down causing Hans to pushes him into the wall as he demands want blood Hans tells him not to alter their plan and when he, Franco, and Fritz were in the elevator, assembled it by putting a magazine in it and attaching the barrel. When the elevator opened at the roof, Karl stated that "nobody kills him but me" and shoots at McClane, nearly knocking McClane off the building. When Karl saw McClane's lighter in the 34th floor vent inside the elevator shaft, he trailed him to a room that was under construction and fired the Steyr AUG at the vent, hoping to hit McClane. Karl then used the muzzle of the AUG to push the vent covers up to find McClane. Karl then uses the AUG on McClane on the 33rd floor and shoots out the glass with it to trap McClane in, since he is barefoot. But McClane gets away by running over the glass shards, much to Karl's fury. After his leader Hans Gruber gets the detonators, Karl is told to smile because they're "back in business". A frustrated Karl goes back to the 30th floor and trashes the wine glass boxes with the butt of his Steyr AUG in anger before walking away.
When McClane finds the C4 explosives and tries to warn Sgt. Al Powell about the terrorist's plans to destroy the roof, Karl arrives and points his AUG rifle to McClane's left cheek with it, forcing him to stop. Before Karl can kill McClane with it, the two get into an intense fight where McClane gets the upper hand by wrapping chains around his neck, seemingly hanging him. After the death of Hans, Karl arrives outside the building and points his AUG at McClane and his wife, Holly, to try and kill them one last time. But Powell shot him several times with his service revolver before he could do so.