SIG 550 – PE90

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 retaining comparable or improved accuracy out to 300 m. The solicitation was narrowed down to two designs: the W+F C42 (developed by the state-owned Waffenfabrik Bern, using both 6.45x48mm and 5.6x45mm cartridges) and the SG 541 (developed by SIG). In 1981, the experimental 6.45mm GP 80 cartridge was rejected in favor of the more conventional 5.6x45mm GP 90 round (with a 4.1 g, steel-jacketed, lead core projectile) that is the Swiss equivalent to NATO’s standard 5.56x45mm cartridge.

Production

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 delivered since then and production for the military has now ceased.

Design

Operating mechanism

SIG SG 550 disassembled into its main constituent groups

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 ergonomics of the rifle provide the shooter with easy access to various controls. The ambidextrous fire selector/safety is manipulated by the thumb of the shooting hand (the 3 bullet burst mode “3″ and the fully automatic “20″ position can be disabled by a rotating safety guard to avert accidentally activating the continuous fire mode). The magazines can be stacked using a connector system.

The gas block integrates an adjustable gas regulator, the front sight with a flip-up night sight post, and the bayonet connector on the flash suppressor.

A loaded 20-round magazine. These are made from a translucent durable polymer allowing the shooter to monitor ammunition levels visually.

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

Rotating diopter drum rear sight. The aperture above the “3″ (denoting the 300 m setting) can be seen.

Tritium-illuminated night sights. Rear sight at notch “1″ with front night sight flipped-up.

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 1.5 mrad or 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.

An SG 550 equipped with the Kern 4×24 telescopic sight

For designated marksman use, the SG 550 is equipped with a Kern 4×24 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.[3] 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.