Complete family of SIG 55x rifles, top to bottom: SIG SG 550 / Stgw.90, SIG SG 551-LB SWAT, SIG SG 551 and SIG SG 552 SWAT

SIG SG 550 / Stgw.90 assault rifle, left side view

SIG SG 551 SWAT compact assault rifle for police/special forces use, fitted with Trijicon ACOG scope, tactical light, front grip and cheek rest

SIG SG 551 SWAT short assault rifle for police/special forces use, fitted with Trijicon ACOG scope, tactical light and front grip; modern production version with ful-length Picatinny rail on top and folding iron sights.

	SIG SG 550 / Stgw.90	SIG SG 551 (SG 551 LB)	SIG SG 552
Caliber	5.56x45mm	5.56x45mm	5.56x45mm
Length (stock open / folded)	998 / 772 mm	833 / 607 mm (924 / 698 mm)	730 / 504 mm
Barrel length	528 mm	363 mm (454 mm)	226 mm
Weight empty	4.05 kg w/o magazine	3.3 (3.4) kg w/o magazine	ca. 3.0 kg w/o magazine
Magazine capacity	20 or 30 rounds	20 or 30 rounds	20 or 30 rounds
Rate of fire	700 rounds/min	700 rounds/min	780 rounds/min

In the late 1970s the Swiss army began the search for a new, smaller caliber rifle to replace old,full-power 7.5mm Stgw.57 (SIG 510) automatic rifle. Initial tests were done with the 5.6 x 48 Eiger and 6.5 x 48 GP80 ammunition. Prototype rifles were developed by SIG (based on their SG-540 design) and by the state-owned Waffenfabrik Bern (W+F). However, the Swiss army selected a slightly improved version of the 5.56 x 45 NATO cartridge as the 5.6 mm GP90, and further testing proved the superiority of the SIG SG-541 rifle over its W+F rival. In 1983, the Swiss Army officially adopted the SIG SG-541 as the Sturmgewehr-90, or Stgw.90 although due to financial reasons production began only in 1986. Currently, the Stgw.90 is a standard Swiss service rifle.
The Swiss army took its last deliveries of the Stgw.90 in the mid-1990s, but these rifles are still offered for export by the international SIGARMS organization, as well as sold for the civilian market in semi-automatic only versions. In export form this rifle is known as the SIG SG-550. "Carbine" and subcompact "Commando" assault rifle versions are available in the form of SIG SG-551 and SIG SG-552, respectively. Civilian versions of the SIG SG-550 and 551 are known as the Stgw.90 PE in Switzerland or SIG 550-SP and 551-SP when sold for export.
The SIG-550 is often referred as the finest 5.56 mm rifle ever made. It is also, not surprisingly, quite expensive.
Recently, SIG Arms has introduced a civilian / law-enforcement offspring of the SG-550, known as SIG 556. This is a semi-automatic rifle, altered to accept M16-type magazines and fitted with "Americanized" furniture.
Basically, the SIG-550 is a somewhat lightened and refined SIG SG-540/541 rifle. SIG SG-550 is gas operated, with gas piston removably attached to the bolt carrier, and with rotating bolt with two massive lugs. The recoil spring is located around the gas piston rod, above the barrel, and the bolt carrier attached to the gas piston rod by the mean of removable charging handle. The most compact weapon in the line, the SIG SG-552, is similar to SIG SG-551 but it has even shorter handguard and barrel. Due to short barrel and gas piston, SG-552 has return spring relocated to the rear part of the receiver, and thus it has an altered bolt carrier and upper receiver.  The gas port has gas regulator with two different open and one closed position (latter for firing rifle grenades). The receiver of all SG-55x series guns is made from stamped steel and has two major parts, upper and lower, which are connected by pushpins. The barrel is screwed into the upper receiver. The trigger unit has a safety/fire selector switch on the left side of the receiver, with 3 settings: safe, semi-auto, full-auto. If desired, additional module could be installed in the trigger mechanism to allow 3-rounds burst mode. Rear sights are drum-type (like those found on Heckler-Koch rifles). On certain current production models rear sight is replaced with full-length Picatinny style rail with folding back-up reat sight. SIG SG-550 has muzzle compensator/flash hider of NATO-standard diameter, so it is possible to launch rifle grenades from the muzzle. The SIG-550 can be fitted with detachable folding bipods under the handguard, and is issued with side-folding, skeletonized polymer buttstock. Every rifle of SIG SG-550 family can be fitted with proprietary, quick detachable scope mount, although current production rifles are usually fitted with one or more picatinny rails. Swiss Stgw.90 are often seen with 4X fixed power scope, export versions can be equipped with commercial telescope sights, ACOG or "red dot" sights, depending on customer preferences. SIG SG-550 also can be fitted with bayonet. Standard magazine capacity for Swiss military Stgw.90 rifles is 20 rounds, as Swiss tactical doctrine calls for accurate semi-automatic fire, reserving full automatic mode only for emergency purposes. Standard magazines can be clamped together for ease of carry, using integral studs on the magazine walls. For those who might require more firepower, SIG also produces 30-round magazines.