Notes: This weapon was developed in response to a Swiss need for a new service rifle to replace the STGW-57; in Swiss service, this assault rifle is called the StG-90. It was adopted in 1984. The weapon was designed for light weight, balance, and accuracy, using plastics for the buttstock, handguard, and pistol grip, as well as the magazines. The stock is folding and skeletonized to reduce weight. The magazines are clear so ammunition levels can be checked, and are equipped with studs and lugs so that up to three of them can be clipped together for rapid changing. The sights consist of day and illuminated night rear sights (one flips the sights to change between the two), and a hooded post-type front sight (also with a tritium inlay). The sights are adjusted for elevation and windage from the rear sight only, and when either the day or night sight is adjusted, the other sight is simultaneously adjusted. There is a STANAG-compatible sight mount for optics of the NATO sort, and this mount can also accept a MIL-STD-1913 rail. The SG-550 is notable for its long 20.8-inch barrel; in addition, a bipod is standard equipment. Various Western-type 40mm underbarrel grenade launchers may be mounted.

The SG-551 is a carbine version of the SG-550, with a shorter 14.6-inch barrel, no bipod, and a rudimentary cheekpiece on the stock, but otherwise identical to the SG-550. A variant of the SG-551, the SG-551 SWAT, is identical except for a redesigned stock and the addition of a permanently-mounted MIL-STD-1913 rail to allow a greater range of sights to be mounted.

The SG-552 is a short-barreled carbine for use by special operations personnel; it also has a permanently-mounted MIL-STD-1913 rail, and can still mount an underbarrel grenade launcher or fire rifle grenades, despite the 10.7-inch barrel. A three-round burst mechanism has been added to the normal selector modes. The barrel is equipped with a muzzle brake instead of a simple flash suppressor. It also has a folding stock, and is known as the Commando. This version was not introduced until 1998.

In addition to these variants, semiautomatic versions of the SG-550 and SG-551 also exist for civilian sales. They are called the SG-550SP and SG-551SP; the SG-551SP version is rarely found outside of Switzerland, however, due to the short length of its barrel and the plethora of government regulations in various countries. The SG-550SP is often found outside Switzerland minus its flash suppressor and sold with 10-round magazines (particularly in the US prior to the sunset of the Assault Weapon Bans).

Finally, the SG-556 is a version of the SG-550 designed for sales in the US, and built in SiGArms’ facilities in the US. Internally, the SG-556 is virtually identical to the civilian versions of the SG-550; however, they have some changes both to suit US firearms regulations and to suit the tastes of American shooters. The SG-556 uses a Vltor 5-position stock, similar in design to that used on the M-4 but more adjustable and with a compartment in the rear large enough to house a standard cleaning kit. The cold-forged barrel is 16 inches long, and it is tipped with a flash suppressor which is similar (but not exactly the same) as that of an M-16A2. The fore-end is coated with ribbed, non-slip rubber. The receiver is topped with a MIL-STD-1913 rail, and three more are on the front of the weapon. Both the front and rear iron sights are military-type and flip down if other optics are mounted. (They can also be removed entirely.) The SG-556 is designed to use standard AR-15/M-16 magazines (the standard SG-550 series uses proprietary magazines). Law enforcement versions have a removable rubber handgrip/fore-end, which can be replaced with a fourth MIL-STD-1913 rail. The SG-556 was introduced at the 2006 SHOT show and began sales the following fall.

Twilight 2000 Notes: There are virtually no SG-552s in existence in the Twilight 2000 timeline which were manufactured as such; however, Swiss gunsmiths often made ad hoc "SG-552s" from existing SG-550s and SG-552s for CQB. The SG-556 does not exist in the Twilight 2000 timeline.