The SG 550 is an assault rifle manufactured by Swiss Arms AG of Neuhausen, Switzerland. SG, as used in the naming of the rifle is an abbreviation for Sturmgewehr, or assault rifle. The SIG SG 550 rifle is based on the earlier 5.56mm SG 540 and is also known as the Fass 90 (Fusil d’assaut 90 / Fucile d’assalto 90) in French / Italian or Stgw 90 in German (Sturmgewehr 90).
SIG SG 550 Development
In 1978, the Swiss Army formulated requirements for a successor to the Stgw 57 battle rifle, commercially known as the SG 510, using the 7.5x55mm GP11 cartridge. The Emphasis was placed on a modular system; 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 in the development of the SIG SG 550 was to reduce the overall weight of the rifle while retaining comparable or improved accuracy out to 300m. 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.
SIG SG 550 Production
In February 1983, the decision to adopt the SG 541 was publicly announced. The designation of the rifle was changed a year later that year in October 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.
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 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.
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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 the spent cartridge casings.
Overall, the operating system is very similar to the AK-47. The bolt and bolt carrier group are very similar to the AK design as is the method of bolt lockup to the barrel breech.
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