Specification	Value
Full text name	Steyr-Mannlicher STM-556 / Rheinmetall-Steyr RS-556 Assault Rifle
Caliber cartridge	5.56x45 NATO, .223 Remington
Action type	select-fire
Trigger type	sa
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

The STM-556 Assault Rifle was first introduced in 2012 by the famous Austrian gun making company Steyr-Mannlicher. In 2016 it was announced that this rifle will be entered into the Bundeswehr (German Army) contest to replace the HK G36 rifles then in service. To participate in this contest, Steyr-Mannlicher teamed up with another famous military contractor, the German-based company Rheinmetall. Their entry is specifically designated as RS-556 (for Rheinmetall-Steyr).

The STM-556 / RS-556 Rifle represents an attempt to merge the now ubiquitous U.S. M4 Carbine‘s ergonomics and layout with certain other advanced features, borrowed from their own famous Steyr AUG-77 Assault Rifle.

This weapon therefore uses the same basic layout as the M4 Carbine, with a two-part receiver, made from aluminum alloy and connected by two cross-pins. The controls are also similar to the M4,except that in this case they are fully ambidextrous.

The key differences between an STM-556 and a standard M4 Carbine lay in the gas system and barrel-receiver interface. The barrel on STM-556 is quick-detachable, with a replacement system based on the proven Steyr AUG. Changing the barrel requires no tools or disassembly, and takes less than a minute. The gas system is also based on the Steyr AUG, with a manual gas regulator with short-stroke gas piston.