The solicitation was narrowed down to two designs: the W+F C42 (developed by the state-owned Waffenfabrik Bern, using both 6.45×48mm and 5.6×45mm cartridges) and the SG 541 (developed by SIG). Another aim was to reduce the overall weight of the rifle while retaining comparable or improved accuracy out to 300 m. 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.

n 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. 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 an assault rifle manufactured by Swiss Arms AG (formerly Schweizerische Industrie Gesellschaft) of Neuhausen, Switzerland (SG is an abbreviation for Sturmgewehr, or 'assault rifle').

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