|Full text name||Steyr-Mannlicher STM-556 / Rheinmetall-Steyr RS-556 assault rifle|
|Caliber cartridge||5.56x45 NATO, .223 Remington|
|Overall length, mm||960|
|Length, folded, mm||860|
|Barrel length, mm||406|
|Weight empty, kg||3,7|
|Magazine capacity, rounds||30|
|Cyclic rate of fire, rounds/min||600-800|
STM-556 assault rifle was first introduced in 2012 by famous Austrian gun making company Steyr-Mannlicher. In 2016 it was announced that this rifle will be entered into Bundeswehr (German army) contest to replace HK G36 rifles now in service. To participate in this contest, Steyr-Mannlicher teamed with another famous military contractor, German-based Rheinmetall company, and their entry is designated RS-556 (as Rheinmetall-Steyr).
Steyr-Mannlicher STM-556 / Rheinmetall–Steyr RS-556 assault rifle is based on US M4 carbine, using same basic layout with two-part receiver, made from aluminum alloy and connected by two cross-pins. All controls are also similar to M4,except that they are made fully ambidextrous. Key differences between STM-556 and standard M4 carbine lay in the gas system and barrel-receiver interface. The barrel on STM-556 is quick-detachable, with replacement system based on the proven Steyr AUG. Change of the barrel requires no tools and no disassembly, and takes less than a minute. The gas system is also based on the Steyr AUG, with manual gas regulator and short-stroke gas piston.