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 included 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 rifle’s overall weight 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 long sight radius of 466 mm (18.3 in). The handguards were also changed and the bipod removed. The SG 551 cannot be used with bayonets 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 fitted 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.