SIG SG 550

The SG 550 is an assault rifle manufactured by Swiss Arms AG (formerly a division of Schweizerische Industrie Gesellschaft now known as Sig Holding AG) of Neuhausen, Switzerland. "SG" is an abbreviation for Sturmgewehr, or "assault rifle". The 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).

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. 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.