SIG 550

From Gunpedia

SG 550
Type	Assault rifle
Place of origin	Switzerland
Service history
In service	1990-present
Used by	See Users
Production history
Designer	SIG
Designed	1970s
Manufacturer	Swiss Arms AG (formerly SIG)
Produced	1986-present
Number built	Approx. 600,000
Variants	See Variants
Specifications
Cartridge	5.6 mm Gw Pat 90 5.56x45mm NATO
Action	Gas operated, rotating bolt
Rate of fire	Approx. 700 rounds/min
Effective range	100 to 400 m sight adjustments
Feed system	5, 20, 30-round detachable box magazine
Sights	Rear: rotating diopter drum with tritium night inserts; front: hooded post with folding night post Template:Convert sight radius (SG 550) Template:Convert sight radius (SG 551)

The SG 550 is an assault rifle manufactured by Swiss Arms AG (formerly Schweizerische Industrie Gesellschaft) of Neuhausen, Switzerland; (SG is an abbreviation for Sturmgewehr, or "assault rifle"). The rifle is based on the earlier 5.56 mm SG 540 and 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).

Contents 1 Development 2 Design details 3 Variants 4 Users 5 Gallery

[edit] Development

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.

[edit] Design details

The SG 550 is a selective fire rifle and features a proven gas-actuated piston-driven long stroke operating system from the SG 540 series of rifles, which uses burnt powder gases vented through a port in the barrel to power 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. Surplus 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 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 Template:Convert 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.^[1] 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.^[2] The basic model of this optical sight was already used on the Stgw 57.^[3]

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.

All rifles are test fired for accuracy and function prior to leaving the factory at the manufacturer's underground 300 m test range; random new rifles out of production are tested. In a 24 single shot string using GP90 ammunition, the group diameter of any individual weapon must be within an Template:Convert group at 300 m, the windage and elevation dispersion must average Template:Convert.^[4]

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 Template:Convert unloaded, length - Template:Convert (has 6 grooves with a Template:Convert rifling twist).

[edit] Variants

The SG 551 carbine has a shorter Template:Convert 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 Template:Convert that enables the use of rifle grenades.

In 1998, the SG 552 Commando carbine was introduced. It has a shorter Template:Convert 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 Template:Convert barrel designed to fire rifle grenades and mount a bayonet.

A new, improved version of the SG 552 has recently been released, the 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.^[5]

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 Template:Convert long barrel with a Template:Convert 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 M16 magazines and an M4 telescoping buttstock. The barrels 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 and the overall length is Template:Convert. Most other parts are interchangeable with the SG 550 series.^[6]

[edit] Users

• Finnish police's special unit Karhu-ryhmä has tested the SG 552 and it is currently fielded by that unit.
• The SG 551 carbine is used by some specialized units of the French Army.^[7]
• The German Federal Police's counter-terrorism unit GSG 9 and the Spezialeinsatzkommando group of the state police both employ the SG 551 and SG 551 LB carbines in a variety of roles.
• Small numbers of the SG 551 LB employed by the 307th Marine Battalion.
• Used by the SAJ (Special Anti-terrorist Unit) version SIG SG 552.
• The Slovak Police Útvar Osobitného Určenia ("special assignments unit") utilizes the SG 551 carbine.
• National police GEO special operations group.
• As of 2008, the SG 550 is the standard service rifle of the Swiss Armed Forces and it is also offered for export.
• Used by some federal agencies.
• Used by the Papal Swiss Guard.^[8]
• Used by the Federal Reserve Police

[edit] Gallery

Caroline-profile-bipode-p1000509.jpg With bipod extended.	Caroline-demontee-p1000529.jpg Disassembled.	Caroline-pontet-p1000527.jpg The fire selector lever.	Caroline-canon-degivrage-p1000523.jpg Flash suppressor and gas regulator.
Caroline-mire-p1000518.jpg Rear sight assembly.	Fass90-bayonette-p1000786.jpg With bayonet.	Caroline-crosse-repliee-p1000515.jpg Stock folded.	Caroline-chargeur-plein-p1000499b.jpg Loaded semi-transparent plastic magazines. The two round protuberances are male parts of the system which allows the magazines to be clipped together (female parts are on the other side).
SIG-552-p1030040.jpg SG 552 Commando version with accessories.	SIG-552-p1030041.jpg The SG 552 with integral Picatinny rail adapter and an Aimpoint red dot sight.	Cartouches-Fass90-p1000785.jpg Every Swiss soldier equipped with the SG 550 assault rifle used to be issued 50 Gw Pat 90 rounds in a sealed box, to be opened only upon alert. The ammunition was intended for use by the militiaman should the need arise while he is en route to join his unit. This concept of personal ammunition has been repealed in 2007 due to safety considerations.

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