This is a Steyr Arms known as the AUG chambered in 223 Remington, model AUGM1MUDEXT.
The iconic bullpup-style Steyr AUG has been one of the most recognizable rifles in the world since its adoption by the Austrian army in 1977 and is now available for civilian use as the Steyr AUG A3 M1" an ambidextrous rifle platform with an adjustable short-stroke" gas-piston operation chambered for 5.56x45mm (.223 Remington).
Features:
Gas-piston operated
Bullpup design
8 Inches shorter than an M4 carbine with a comparable-length barrel
