Sig 552 LB

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 (SIG510) automatic rifle. Initial tests were done with the 5.6 x48 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, orStgw.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 ex port 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 SIGSG-551 and SIG SG-552, respectively. Civilian versions of the SIGSG-550 and 551 are known as the Stgw.90 PE in Switzerland or SIG 550-SPand 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 asemi-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 pistonremovably attached to the bolt carrier, and with rotating bolt with two massive lugs. The recoil spring is located around the gas piston rod, above thebarrel, and the bolt carrier attached to the gas piston rod by the mean of removable charging handle. The most compact weapon in the line, theSIG SG-552, is similar to SIG SG-551 but it has even shorter hand guard 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 majorparts, upper and lower, which are connected by push pins. The barrel is screwed into the upper receiver. The trigger unit has a safety/fireselector 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. Rearsights 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. SIGSG-550has muzzle compensator/flash hider of NATO-standard diameter, so it is possible to launch rifle grenades from the muzzle. The SIG-550 can befitted 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 befittedwith 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.

The carbine version of the SIG-550 is called SG-551and has shorter barrel. Quite recently, SIG also introduced a "long barrel" version of the SG-551, which is known as SIG SG-551LB. SIG-551can't fire rifle grenades.

Source : http://world.guns.ru/assault/switch/sig-550--551--552-e.html