Operating mechanism
The SG 550 is a selective fire 5.56x45mm NATO assault rifle firing from a closed bolt. It has a gas-actuated piston-driven long stroke operating system derived from the SIG 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 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 the spent cartridge casings.
Features
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"—fully automatic fire. The trigger is enclosed in a pivoting trigger guard which can be folded down to the left or right side allowing for unhindered operation with winter gloves. The trigger pull is approximately 35 N (7.9 lbf)
The firearm is fed by lightweight 20-round box magazines, 30-,10- and 5-round magazines are also available. The magazines are molded from a translucent polymer and can be locked together using studs in order to facilitate quicker reloading; Jungle style. The empty weight of a 20-round magazine is 95 g (3.4 oz) and 110 g (3.9 oz) for a 30-round magazine.
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. Alternatively, a left-handed shooter may release the bolt by pulling the rubber-coated charging handle to rear a short distance.
The SG 550 has a side-folding skeletonized buttstock (folds to the right side of the receiver) and a lightweight aluminium bipod that folds into grooves in the lower handguard. The hinged stock is firmly locked in the folded position by a socket in the butt which clips into a plastic stud on the handguard; a firm pull will release the stock which is then swung into the closed position and locked by a button catch. A collapsible side-folding stock is also available. The stock, pistol grip, and handguards are made of a high-strength polymer, and are produced in either green or black colour options. 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. 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.
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) or 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 is optimized for Swiss military GP 90 ammunition. An export-oriented barrel with a 178 mm (1:7 in) twist rate is also available, designed to stabilize 5.56x45mm NATO rounds with the heavier SS109 projectile.
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 were tested of a machine rest. In a 24 single shot string starting with a cold weapon and using GP 90 ammunition, the 50% windage and elevation dispersion of any individual weapon must have been within an 11 cm (4.3 in) group at 300 m, the 50% windage and elevation dispersion must have averaged 7 cm (2.8 in).
Sights
The SG 550 series rifles have a 540 mm (21.3 in) long sight radius and are equipped with iron sights adjustable for both windage and elevation. The sights consist of a rear, rotating diopter drum soldered to the receiver and a hooded front post installed in the gas block. 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 200, 300 and 400 m firing ranges. The 400 m setting has a removable iris for sportive shooting. The sights are adjustable via micrometer screws 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 inserts fitted laterally on each side of the notch and additionally in a flip-up post attached to the foresight. When firing rifle grenades the front sight hood is aligned with the uppermost edge of the grenade's warhead, this provides an estimated point of impact up to 75 m. The rifle grenades intended for this purpose were FN/Luchaire Type 58-N bullet-though anti-tank grenades.
The rifle's sights were placed relatively low over the bore axis to ensure a minimal silhouette when firing from the prone position (the shooter presents a 20% smaller target than with the Stgw 57), but high enough to provide comfort and speed when shouldering the rifle in the kneeling and offhand shooting positions. The design of the shoulder stock also facilitates the rapid alignment of the shooting eye with the rifle's line of sight.
For designated marksman use, the SG 550 is equipped with a Kern 4x24 telescopic sight on a quick-detachable mount. The sight weighs 730 g (26 oz) 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, a tritium-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.
The upper receiver can accept quick-detachable rails and adapters used to mount optics (STANAG 2324 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—a rear aperture sight which folds down into a recess in the rail and a folding front blade.
Accessories
Both the rifle and carbine come standard with a spare magazine, sling, cleaning kit and a loading aid for rapid magazine filling.
The firearm will accept SIG's 40 mm GL 5040/5140 grenade launcher (Swiss military designation: 40 mm Gewehraufsatz 97), which is mounted under the barrel via an eccentric latch and replaces the lower handguard. The grenade launcher is a single-shot breech-loaded weapon that is supplied with a leaf sight that attaches to the rifle's rear sight base and enables accurate firing out to 200 m. The lightweight aluminium launcher weighs 1.7 kg (3.7 lb) unloaded, and is operated independently of the rifle. It can use a wide array of 40x46mm grenades, including extended range high-pressure types and non-lethal baton or anti-riot projectiles.
A Stgw 90 bayonet can also be mounted to the rifle. The bayonet has an overall length of 310 mm and a muzzle ring diameter of 22 mm. The 177 mm long blade is single-edged and it has no fuller. The bayonets are manufactured exclusively for the Swiss Army by Wenger and Victorinox (before the two companies merged). With a proper lug adaptor, the rifle will also accept a NATO-pattern KCB-77 (made originally by Carl Eickhorn of Solingen, West Germany) or the American M9.
Variants
SG 551
The SG 551 carbine has a short pattern 363 mm (14.3 in) barrel, gas tube and piston compared to the SG 550. The SG 551 series rifles have a 466 mm (18.3 in) long sight radius. 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, and 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 and a bayonet.
SG 552 Commando
The compact SG 552 Commando carbine was revealed in 1998. 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 SG 552 series rifles have a 360 mm (14.2 in) long sight radius. 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.
SG 553
The SG 553 is an improved version of the SG 552 and has been released in 2009. 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 standard SG 550/551 charging handle. The SG 553 series rifles have a 339 mm (13.3 in) long sight radius. A long barrel version of the SG 553 known as the SG 553 LB incorporates a 347 mm (13.7 in) barrel with provision to fire rifle grenades and support a bayonet. Further factory options for the SG 553 rifle series are an integrated receiver Picatinny rail and an adjustable butt stock.
SG 550 Sniper
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 N (7.9 lbf) to 15 N (3.4 lbf), a heavy, hammer-forged 650 mm (25.6 in) long barrel with a 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 cheek piece and a spacer system on the butt, the ergonomic pistol grip's angle of inclination can be regulated and the bipod features a height and cant adjustment mechanism. The rifle is no longer in production.
Sporting variants
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 and SIG Sport rifles. The SG 550 series is available with either 178 mm 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 178 mm (1:7 in) twist rate. The ordnance GP 90 ammunition is optimized for use with the original Swiss 254 mm (1:10 in) rifling twist rate.
Due to import restrictions, the American civilian market required a partially American-made version assembled by SIG SAUER, Inc. (not to be confused with the Swiss-German entity) in Exeter, New Hampshire. The SIG556 is a commercial variant of the SG 551 designed to meet these requirements. The 556 lacks full-auto capability and the overall length is 940 mm (37.0 in). The main difference is a new aluminum lower receiver that accepts M16 STANAG magazines and an M4 telescoping buttstock. The barrel's twist rate is 178 mm (1:7 in). Most other parts are interchangeable with the SG 550 series.
There are many variants of this rifle offered for sale. The first variant was sold with an aluminum Picatinny rail on the upper receiver and a series of plastic rails on the handguard. The market pushed SIG to produce the rifle with the slimmer profile 551-type handguards and a hooded front sight; this version is marketed as the SIG556 Classic. Several folding stock models have been released as well as variants with railed forend combinations. The most recent major variant is a SIG556 DMR. This features a 21 in (533.4 mm) long barrel without flash suppressor, upgraded, a match type trigger, Magpul PRS stock and older style plastic handguards. SIGP556 pistol or "factory SBR" variants with 10 in barrels are also available.
The SIG522LR is a .22-caliber sporting rifle styled after the SG 551. It uses a simple blowback semi-automatic operating system and its barrel has a 406 mm (1:16 in) twist rate. Due to its operating principle, the rifle has no mechanical commonality with other SG 550 variants. The SIG522 accepts commonly available AR-style .22-caliber conversion magazines.
| SG 550 | |
|---|---|
| Type | Assault rifle |
| Place of origin |  Switzerland |
| Service history | |
| In service | 1990–present |
| Used by | Brazil, France, Germany, Indonesia, India, Malaysia, Slovakia Spain, Switzerland, United States |
| Production history | |
| Designer | SIG |
| Designed | 1970s |
| Manufacturer | Swiss Arms AG (formerly SIG) |
| Produced | 1986–present |
| Number built | 600,000+ |
| Specifications | |
| Weight | SG 550: 4.1 kg (9.04 lb) SG 551: 3.40 kg (7.5 lb) SG 552 Commando: 3.2 kg (7.1 lb) SG 553: 3.2 kg (7.1 lb) SG 550 Sniper: 7.02 kg (15.5 lb) |
| Length | SG 550 (stock extended): 998 mm (39.3 in) SG 550 (stock folded): 772 mm (30.4 in) SG 551 (stock extended): 833 mm (32.8 in) SG 551 (stock folded): 607 mm (23.9 in) SG 552 Commando (stock extended): 730 mm (28.7 in) SG 552 Commando (stock folded): 504 mm (19.8 in) SG 553 (stock extended): 730 mm (28.7 in) SG 553 (stock folded): 501 mm (19.7 in) SG 550 Sniper (stock extended): 1,130 mm (44.5 in) SG 550 Sniper (stock folded): 905 mm (35.6 in) |
| Barrel length | SG 550: 528 mm (20.8 in) SG 551: 363 mm (14.3 in) SG 552 Commando: 226 mm (8.9 in) SG 553: 227 mm (8.9 in) SG 550 Sniper: 650 mm (25.6 in) |
| | |
| Cartridge | 5.6mm Gw Pat 90 (5.56x45mm NATO) |
| Action | Gas operated, rotating bolt |
| Rate of fire | Approx. 700 rounds/min |
| Muzzle velocity | SG 550: 911 m/s (2,989 ft/s) SG 551: 850 m/s (2,788.7 ft/s) SG 552 Commando: 725 m/s (2,378.6 ft/s) SG 550 Sniper: 940 m/s (3,084.0 ft/s) |
| Effective range | 100–400 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 540 mm (21.3 in) sight radius (SG 550) 466 mm (18.3 in) sight radius (SG 551) |
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