The SIG551-A1 is Sig Sauer’s third semi-automatic rifle sold in the U.S. market, based on the SGW550. Sig Sauer’s lineup now includes the SIG551-A1 and SIG556 chambered in 5.56mm NATO, and the SIG556R in 7.62x39mm.
The SIG551-A1 is the rifle most closely patterned after the SGW550, which was adopted in 1990 as the standard infantry rifle for the Swiss Army and citizen militia. Differences between the two rifles are few: the SGW550 has a forged steel trigger housing and select-fire mechanism, a 20.5-inch barrel with a slower rate of twist than the SIG551-A1 (with its 16-inch barrel and 1-in-7-inch twist), a bayonet lug, tritium night sights and a handguard with an integral folding bipod. Because of the longer barrel, bipod and steel trigger housing, the SGW550 is also much heavier than the SIG551-A1.
The SIG551-A1 shares many features of the SIG556, which was the company’s first semi-automatic rifle sold in the U.S. in many years, but departs from that design in a few areas. The most significant difference is the use of a more substantial forged aluminum trigger housing assembly modeled closely after the SGW550 and the same magazine and loading technique as the SGW550. In contrast, the SIG516 has a stamped-steel trigger housing assembly and uses M16 magazines.