SIG SG 550

SG 550
	; 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
Weight	4.1 kg (9.04 lb) (SG 550); 3.40 kg (7.5 lb) (SG 551); 3.2 kg (7.1 lb) (SG 552 Commando); 7.02 kg (15.5 lb) (SG 550 Sniper)
Length	998 mm (39.3 in) stock extended / 772 mm (30.4 in) stock folded (SG 550); 833 mm (32.8 in) stock extended / 607 mm (23.9 in) stock folded (SG 551); 730 mm (28.7 in) stock extended / 504 mm (19.8 in) stock folded (SG 552 Commando); 1,130 mm (44.5 in) stock extended / 905 mm (35.6 in) stock folded (SG 550 Sniper)
Barrel length	528 mm (20.8 in) (SG 550); 363 mm (14.3 in) (SG 551); 226 mm (8.9 in) (SG 552 Commando); 650 mm (25.6 in) (SG 550 Sniper)
Cartridge	5.6 mm Gw Pat 90; 5.56x45mm NATO
Action	Gas operated, rotating bolt
Rate of fire	Approx. 700 rounds/min
Muzzle velocity	980 m/s (3,215 ft/s) (SG 550); 850 m/s (2,788.7 ft/s) (SG 551); 725 m/s (2,378.6 ft/s) (SG 552 Commando); 940 m/s (3,084.0 ft/s) (SG 550 Sniper)
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; 500 mm (19.7 in) sight radius (SG 550); 426 mm (16.8 in) sight radius (SG 551)

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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).

[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

A Swiss Army soldier with the SG 550.

The SG 550 disassembled.

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 that used in the SG 540. A spring-loaded extractor is incorporated into the bolt, while a fixed protrusion on one of the receiver's internal guide rails 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 winter gloves. 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 1 in 254 mm (1:10 in) rifling twist rate optimized for GP 90 and 5.56x45mm M193 ammunition. An export-oriented barrel with a 1 in 178 mm (1:7 in) twist rate is also available, designed to stabilize 5.56x45mm NATO rounds with the heavier SS109 projectile.

The SG 550 series rifles are equipped with adjustable iron sights corrected for both windage and elevation. The sights consist of a rear, rotating diopter drum and a hooded front post. The rear sight has an open notch setting marked "1" designed for immediate firing up to 100 m but also contains apertures with settings "2", "3" and "4" corresponding to firing ranges 200, 300 and 400 m. The 400 m setting has a removable iris for sportive shooting. The sights are adjustable with windage and elevation increments of 4.5 cm (1.8 in) at 300 m. For night use, the dedicated "1" notch setting in the rear sight drum is provided with two self-luminous tritium-powered night inserts fitted laterally 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 compliant). The scope mounting system 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) offer several types of quick-release scope mounts and Picatinny rails. A version of the rifle with an integral receiver-mounted Picatinny rail is also offered; in this configuration the weapon is fitted with flip-up emergency battle sights comprised of a rear aperture sight which folds down into a recess in the rail and a folding front blade.

For designated marksman use, the Swiss Army SG 550 is equipped with a quick-detachable Kern 4x24 telescopic sight. The sight weighs 730 g and includes a variety of features, such as STANAG 2324/MIL-STD-1913 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.^[1] The basic model of this optical sight was already used on the Stgw 57.^[2]

The standard SG 550 has a side-folding skeletonized buttstock (folds to the right side of the receiver) and a lightweight integral bipod that folds into grooves in the lower handguard. A collapsible side-folding stock has also recently become available. 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; external steel surfaces are finished with a ceramic-reinforced enamel coat known as Ilaflon.^[3] The gas system's components are made of stainless steel. The barrel, bolt, bolt carrier, and firing pin are all made with steel that has been gas nitrided, hardened and tempered. The bolt and carrier, along with most other components internal to the receiver undergo a phosphating process.

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 GP 90 ammunition, the group diameter of any individual weapon must be within an 11 cm (4.3 in) group at 300 m, the windage and elevation dispersion must average 7 cm (2.8 in).^[4]

Both the rifle and carbine come standard with a spare magazine, sling, cleaning kit and a loading aid for rapid magazine filling. The firearm is also compatible with SIG's 40 mm GL 5040/5140 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 that enables accurate firing out to 200 m. The launcher weighs 1.7 kg (3.7 lb) unloaded, length – 305 mm (12.0 in) (has 6 right-hand grooves with a 1 in 1,200 mm rifling lead).

[edit] Variants

A semi-automatic version of the SG 551 carbine with the stock folded.

The SG 552 Commando version with integral Picatinny rail and accessories.

The SG 551 carbine has a short pattern 363 mm (14.3 in) 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 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 mount used with a wide array of sights, 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 454 mm (17.9 in) barrel that enables the use of rifle grenades.

In 1998 the SG 552 Commando carbine was introduced. It has a shorter 226 mm (8.9 in) 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 that of 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 346 mm (13.6 in) barrel with provision to fire rifle grenades and support 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 wrapped around the piston rod like in the SG 550/551 models, which address several reliability issues encountered in the SG 552 and also allows the usage of the "normal" charging handle. It is available in both standard, short and LB barrel lengths and with 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 650 mm (25.6 in) long barrel with a 1 in 254 mm (1:10 in) 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 1 in 178 mm or 1 in 254 mm (1:7 and 1:10 in) twist rate barrels. Rifles designated SG 55x-1 have a 1 in 254 mm (1:10 in) twist rate, while models marked SG 55x-2 have a 1 in 178 mm (1:7 in) twist rate. The Swiss Gw Pat 90 ammunition is optimized for use with the original Swiss 1 in 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 barrel's twist rate is 178 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 sight. The SIG 556 lacks full-auto capability and the overall length is 940 mm (37.0 in). Most other parts are interchangeable with the SG 550 series.^[6]

[edit] Users

Argentina: GEOF counter-terrorism unit uses both the SG 551 and SG 552 Commando.
Finland: Finnish police's special unit Karhu-ryhmä has tested the SG 552 and it is currently fielded by that unit.
France: The SG 551 carbine is used by some specialized units of the French Army.^[7]
Germany: 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.
Jordan: Jordanian police use the SG 550 Sniper in small numbers.
Romania: Small numbers of the SG 551 LB employed by the 307th Marine Battalion.
Serbia: SG 552 version used by the SAJ anti-terrorist unit.
Slovakia: The Slovak Police Útvar Osobitného Určenia ("special assignments unit") utilizes the SG 551 carbine.
Spain: National police GEO special operations group uses both the SG 551 SWAT and SG 552.^[8]^[9]
Switzerland: As of 2008, the SG 550 is the standard service rifle of the Swiss Armed Forces.^[10]
United States: Used by some federal agencies and the Federal Reserve.
Vatican City: Used by the Papal Swiss Guard.^[11]

[edit] Gallery

With bipod extended.	The fire selector lever.	Flash suppressor and gas regulator.	Rear sight assembly.
With bayonet.	Stock folded.	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).	The SG 552 with integral Picatinny rail adapter and an Aimpoint red dot sight.
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.