SIG SG 550
The SIG SG 550 is an assault rifle manufactured by SIG Sauer AG in Switzerland. "SG" is an abbreviation for Sturmgewehr. The SG 550 was co-developed by Sig Sauer AG and Italian firearm manufacturer Beretta, and is based on the earlier predecessor, the SIG SG 540, chambered in 5.56×45mm NATO.
History
Development
In 1978, the Swiss armed forces formulated requirements for a successor to the Stgw 57 battle rifle using the 7.5×55mm GP 11 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, command staff, vehicle crews, special forces 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 and the SG 541. In 1981, the experimental 6.45mm GP 80 cartridge was rejected in favour of the more conventional SIG 5.6×45mm Gw Pat 90 round that is the Swiss equivalent to NATO's standard 5.56×45mm cartridge.Production
In February 1983, the decision to adopt the SG 541 was publicly announced. Production began in 1986 and four years later 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.Regulation In Canada
On 27 February 2014, the Canadian semi-automatic "Classic Green" sporting rifle, also known as the "Swiss Arms PE 90", was re-classified as a "prohibited weapon". The rifle had been popular with hunters and gun enthusiasts, who until February 2014, only required a possession and acquisition licence to obtain the rifle.In 2013, a gun dealer had imported some PE 90 rifles from Switzerland and brought them to Canada for sale. After some were sold, it was alleged that the rifles in question were of a variant not allowed in Canada. Upon investigation and examination by the RCMP, this was proven inconclusive, however, the RCMP went further and decided the versions in Canada were too close to the PE90 and reclassified the rifles and declared the entire model line prohibited, with possible confiscation for destruction. This caused outrage amongst owners and lobbyists, who felt that the RCMP exceeded their authority, and that such policy changes should be enacted by legislation.
The National Firearms Association of Canada considered pursuit of the matter through the legal system. On 31 July 2015, the Canadian government overturned the reclassification and returned the Swiss rifles to the original classifications.
On 1 May 2020, in the wake of the Nova Scotia attacks, the rifles were again reclassified as Prohibited Firearms via Order in Council.
Design details
Operating mechanism
The SG 550 is a selective-fire 5.56×45mm 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. 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 3-round 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 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.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. The empty weight of a 20-round magazine is and 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 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 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 or attach a knife bayonet. The rifled barrel has 6 right-hand grooves and the Swiss Army specification 254 mm rifling twist rate is optimized for Swiss military GP 90 ammunition. An export-oriented 5.56×45mm NATO barrel configuration with a 178 mm twist rate is also available, to adequately stabilize the relatively long NATO L110/M856 5.56×45mm NATO tracer 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 on a machine rest. In a 24 single shot string starting with a cold weapon and using GP 90 ammunition, the R50 or 50% windage and elevation dispersion of any individual weapon must have been within an group at 300 m, the 50% windage and elevation dispersion must have averaged. The employed circular error probable method cannot be converted and is not comparable to US military methods for determining rifle accuracy. When the R50 results are doubled the hit probability increases to 93.7%.
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.
Sights
The SG 550 series rifles have a long sight radius and are equipped with iron sights adjustable for both windage and elevation. The sights are similar to those on some Heckler & Koch weapons, such as the HK G3 or HK MP5. 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 0.15 mil, or at 100 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.For designated marksman use, the SG 550 is equipped with a Kern 4×24 telescopic sight on a quick-detachable mount. The sight weighs and includes a variety of features, such as STANAG 2324/MIL-STD-1913 compliant mounting components, a Bullet Drop Compensation 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. 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 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.