The SIG SG 551 is a variant of the swiss assault rifle SIG SG 550. In 1978, the Swiss Army formulated requirements for a successor to the Stgw 57 battle rifle (known commercially as the SG 510) using the 7.5×55mm Schmidt Rubin 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-ownedWaffenfabrik Bern, using both 6.45×48mm and 5.6×45mm cartridges) and the SG 541 (developed by SIG). In 1981, the experimental6.45mm GP 80 cartridge was rejected in favor of the more conventional 5.6×45mm GP 90 round (with a 4.1 g, tombac-jacketed, lead core projectile) that is the Swiss equivalent to NATO’s standard 5.56×45mm cartridge.
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 6×42 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.