The SIG 556 semi-automatic rifle was announced in 2006 as a rifle for civilian and law-enforcement use.It is produced by international SIGARMS (now SIG Sauer) arms corporation and intended primary for US market. SIG 556 is based on SIG SG 550 assaul trifle, which is in use by Swiss army, and is limited to semi-automatic fire only, thus being legal in most countries that permit civilians to own rifles. SIG Sauer also produces a SIG 556 SWAT semi-automatic rifle which has a different butt stock and a 4-rail forend.
The SIG 556 semi-automatic rifle is gas operated, using short-stroke gaspiston located above the barrel. Gas system has manual gas regulator with two settings for normal and harsh conditions. The barrel is locked using rotary bolt with two massive lugs. Receiver is redesigned from original SIG SG 550 rifle, and features stamped steel upper part mated to aluminum alloy lower part which houses a trigger unit and a redesigned magazine housing which accepts all M16 type magazines. A bolt stop device is incorportaed into design. Manual safety is ambidextrous and located above the pistol grip. As a standard, rifle is shipped with 30-round magazine. In basic configuration SIG 556 has no iron sights (adjustable folding ironsights are optional). Top of receiver is fitted with Picatinny rail for easy installation of various optical or red-dot sights; additional rails are installed on the forend. Buttstock is of non-folding, telescoped adjustable type.