The SIG556 is a assault rifle based on the SG550. The 556 includes full-auto capability and the overall length is 940 mm (37.0 in). One difference is a new aluminum lower receiver that accepts M16 STANAG magazines and an M4 telescoping buttstock. The barrel's twist rate is 178 mm (1:7 in). The SG 556 is a selective fire 5.56×45mm NATO assault rifle firing from a closed bolt. As of now, it is being used mainly by Alliance of Nations (mainly the Earth Kingdom).