SIG Sauer developed the SIG 556 to comply with U.S. import restrictions by creating a partially American-made version of the Swiss SIG SG 550. This model was intended for the civilian market and lacked full-auto capability. Key features included a 37" overall length, a 1:7 twist rate barrel, and an aluminum lower receiver compatible with M16 STANAG magazines, along with an M4-style telescoping stock. The initial version of the SIG 556 came equipped with an aluminum Picatinny rail on the upper receiver and plastic handguards. In response to market demand, SIG introduced the SIG 556 Classic, which featured slimmer 551-style handguards and a hooded front sight for a more traditional look. Other significant variants included the SIG 556 DMR, which had an 18" barrel for enhanced accuracy, and the SIG P556, a pistol variant with a 10" barrel designed for more compact use. Additionally, SIG Sauer released the SIG 556R, chambered in 7.62×39mm and designed to accept AK-pattern magazines, although it initially faced reliability issues that were later resolved. The SIG 556xi series was introduced in 2014, bringing modularity and enhanced features. Despite its popularity, SIG discontinued the 556, 556R, and 556xi models as of May 2017.