Due to import restrictions, the American civilian market required a partially American-made version assembled by SIG SAUER, Inc. in Exeter, New Hampshire. The SIG556 is designed to meet these requirements. The 556 lacks 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).
There are many variants of this rifle offered for sale. The first variant was sold with an aluminum Picatinny rail on the upper receiver and a series of plastic rails on the handguard. The market pushed SIG to produce the rifle with the slimmer profile 551-type handguards and a hooded front sight; this version is marketed as the SIG556 Classic. Several folding stock models have been released as well as variants with railed forend combinations.