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The SIG SG-550 is an assault rifle manufactured by Swiss Arms AG (formerly Schweizerische Industrie Gesellschaft) of Neuhausen am Rheinfall|Neuhausen, Switzerland; (SG is an abbreviation for Sturmgewehr, or "assault rifle"). It is also known as the Fass 90 (Fusil d'assault 90 / Fucile d'assalto 90) in French/Italian or Stgw 90 in German (Sturmgewehr 90).
In 1978, the Swiss Army formulated requirements for a successor to the Stgw 57 battle rifle (known commercially as the SG 510) using the 7.5x55mm GP11 cartridge. Emphasis was placed on modularity, the weapon family was to include several variants of the base design, including a compact carbine (that would be issued to rear-echelon and support troops, commanding staff, vehicle crews, special operations personnel and paratroopers). Another aim was to reduce the overall weight of the rifle while maintaining comparable or improved accuracy out to 300 m. The solicitation was narrowed down to two divergent designs: the W+F C42(developed by the Federal Arms Factory in Berne, using both 6.45x48mm and 5.6x45mm cartridges) and the SG 541 (developed by SIG in 5.6x45mm). In 1981, the experimental 6.45 mm cartridge was rejected in favor of the more conventional 5.6x45mm Gw Pat 90 (with a 4.1 g, steel-jacketed, lead core projectile) that is the Swiss equivalent to NATO's standard 5.56x45mm cartridge. In February 1983, the decision to adopt the SG 541 was publicly announced (the designation of the rifle was changed in October of the following year to SG 550, while the carbine version became known as the SG 551). The rifle was officially accepted into service in 1990 (hence the military designation Stgw 90). Over 600,000 rifles have been manufactured since then and production continues as of 2008.
The SG 550 is a select-fire rifle, and is based on the earlier 5.56x45mm NATO SIG 540. The standard SG 550 has a side-folding buttstock (folds to the right) and a lightweight, integral bipod that folds under the handguard. The stock, pistol grip and handguards are made of a high-strength polymer. The steel receiver housing and several other components are manufactured using stamping and welding.
It features a gas-actuated piston-driven long stroke operating system from the SG 540 series of rifles, which uses gas produced from the fired cartridge due to the burning propellant, vented through a gas port in the barrel to cycle the weapon's moving parts. Once inside the gas cylinder, propellant gases pass through an L-shaped channel machined in the piston head and are directed forward towards the gas valve. The pressure build-up in front of the piston head pushes the piston and bolt carrier rearward. As the piston is driven back, the gas port and the L-shaped channel move out of alignment, cutting off the supply of gas to the cylinder.
Excess gas and powder residues are evacuated through an exhaust port in the gas cylinder. This system ensures that only the precise volume of gas required to overcome the mass and resistance of the rifle's moving assembly is admitted from the barrel.
The manually adjustable gas valve has two settings: one for normal operation, and the second setting for use in the presence of heavy fouling or icing. The rifle fires from a closed bolt. The rotary bolt locking mechanism consists of two steel locking lugs that engage locking recesses in the breech and is identical to the SG 540. A spring-loaded extractor is incorporated into the bolt, while a fixed protrusion in the receiver housing ejects spent cartridge casings.
The rifle is hammer fired and has a trigger mechanism with an ambidextrous safety and fire selector switch that has 4 settings: "S" - safe, "1" - single fire, "3" - 3 round burst and "20" - automatic fire. The trigger is enclosed in a pivoting trigger guard, which can be folded down to the left or right side allowing for operation with mittens. The firearm is fed by standard Swiss 20-round lightweight box magazines. The magazines are molded from a transparent polymer and can be locked together using studs in order to facilitate quicker reloading. A bolt hold-open device locks the bolt carrier assembly open after expending the last cartridge from the magazine and is released by lifting the bolt catch lever located on the left side of the receiver.
The heavy, cold hammer-forged barrel is screwed into the receiver and is equipped with a slotted "bird cage" type flash suppressor that is also used to launch rifle grenades (using standard, live ammunition) and attach a knife bayonet (the bayonet is supported by a lug located at the base of the gas block). The rifled barrel has 6 right-hand grooves and the Swiss Army specification 254 mm (1:10 in) rifling twist rate optimized for GP 90 and 5.56x45mm M193 ammunition. An export-oriented barrel with a 177.8 mm (1:7 in) twist rate is also available, designed to stabilize 5.56x45mm NATO rounds with the heavier SS109 projectile. Rifles designated SG 55x-1 have a 254 mm (1:10 in) twist rate, while models marked SG 55x-2 have a 177.8 mm (1:7 in) twist rate.
The barrel has a 14mm x 1mm left-hand threading pitch for flash suppressors.
The sight consist of a rear, rotating diopter drum and a hooded front sight post. The rear sight has an open sight marked "1" ranged for firing up to 100 m. The "2", "3" and "4" correspond to the ranges up to 200 m, 300 m and 400 m and consist of a peephole. The 400 m setting also has a removable peephole which can be fitted with an aperture for sportive shooting. The iron sights are adjustable with windage and elevation increments of at 300 m. For night use, a dedicated 100 m notch setting in the rear sight drum is provided that contains two self-luminous tritium-powered night inserts mounted on each side of the notch and additionally in a flip-up post attached to the foresight. The upper receiver can accept quick-detachable rails and adapters used to mount optics (STANAG system).
The system for attaching aiming optics mounting systems consists of a centering hole located on the front face of the rear sight assembly and a dovetail-like mounting point at the front end of the receiver. Swiss Arms (respectively Brügger & Thomet) offers several types of quick-release scope mounts and Picatinny rails. For example every Swiss Army SG 550 rifle can be equipped for designated marksman use with a quick-detachable Kern 4x24 telescopic sight. The sight weighs 730 g and includes a variety of features, such as STANAG 2324/Picatinny rails compliant mounting components, a Bullet Drop Compensation (BDC) elevation adjustment knob for ranges from 100 to 600 m, an illuminated reticle that enables target acquisition in low-light conditions and a diopter eyesight correction adjustment. Included with the sight is a lens hood for mounting on the ocular that reduces image quality-impairing stray light and a gray filter for glare reduction. The basic model of this optical sight was already used on the Stgw 57.
Both the rifle and carbine come standard with a spare magazine, sling, cleaning kit and speed loading device. The firearm is also compatible with SIG's 40 mm GL 5010/5140 grenade launcher, which is mounted under the barrel and replaces the lower handguard. The grenade launcher comes with a leaf sight that attaches to the rifle's rear sight base. The launcher weighs unloaded, length - (has 6 grooves with a (rifling twist).
The SIG SG-551 carbine has a shorter barrel, gas tube and piston compared to the SG 550. The handguards were also changed and the bipod removed. The SG 551 cannot be used with a bayonet or to fire rifle grenades. The SG 551 comes in several specialized variants designed for use with security and special forces. Among those variants are: the SG 551-1P police carbine (designed to engage point targets out to 300 m, equipped with a Hensoldt 6x42 BL telescopic sight and detachable cheek riser), the SG 551 SWAT carbine (coated with a corrosion-resistant finish and equipped with an optical sight, for example Trijicon's ACOG 3.5x35; this variant can also accept mission-critical accessories such as a bipod, laser pointer or tactical light) and the SG 551 LB carbine with an extended barrel that enables the use of rifle grenades.
In 1998, the SIG SG-552 Commando carbine was introduced. It has a shorter barrel (with an open, 3-prong flash suppressor) and gas tube, ventilated handguards and a redesigned bolt carrier group that was integrated with the piston rod to form a single moving assembly. The return mechanism has been moved to the rear of the receiver housing and its recoil spring is guided in a way analogous to the AK-47, on a steel guide rod (later models feature a polymer guide rod) resting against the lower receiver's rear surface under tension of the compressed recoil spring. Like the SG 550/551, this model can accept rails and accessories enabling the use of optics. A long barrel version of the SG 552 known as the SG 552 LB incorporates a barrel designed to fire rifle grenades and mount a bayonet.
A new, improved version of the SG 552 has recently been released, the SIG SG-553. Even though it mostly resembles the SG 552, the SG 553 has one key advantage, the recoil spring is now located around the piston rod like in the SG 550/551 models, which also allows the usage of the "normal" charging handle. It is available in both standard, short and LB barrel lengths and an optional integrated receiver Picatinny rail.
Another member of the SG-550 family is the SG-550 Sniper variant designed specifically for Swiss security forces. This accurized rifle has a refined two-stage trigger (the pull force was reduced from 35 to 15 N), a heavy, hammer-forged long barrel with a rifling twist rate (it has no flash hider) and is used exclusively with telescopic sights. The new folding stock has an adjustable cheekpiece and a spacer system on the butt, the ergonomic pistol grip's angle of inclination can be regulated and the bipod features a height adjustment mechanism. The rifle is no longer in production.
The SG 550/551/552/553 are also available in semi-automatic only configurations, intended for the civilian shooting market. Among these variants are the SG 550/551/552 SP, PE 90, SIG Sport and Classic Green rifles. The SG 550 series is available with either 177.8 or 254 mm (1:7 and 1:10 in) twist rate barrels. Rifles designated SG 55x-1 have a 254 mm (1:10 in) twist rate, while models marked SG 55x-2 have a 177.8 mm (1:7 in) twist rate. The Swiss Gw Pat 90 ammunition is optimized for use with a 254 mm (1:10 in) rifling twist rate.
Due to import restrictions, the American civilian market required a partially American-made version. The SIG 556 is a commercial variant of the SG 551 designed to meet these requirements. The main difference is a new aluminum lower receiver that accepts AR-15 STANAG magazines and an AR-15 telescoping buttstock. The barrel's twist rate is 177.8 mm (1:7 in). Picatinny rails are installed on the receiver and incorporated into the foregrip for optics and accessories. The diopter sights of the rest of the SG 550 family have been replaced with simpler front and rear flip-up sights. The SIG 556 lacks full-auto capability. Most other parts are interchangeable with the SG 550 series.