Development of the SIG SG 550 assault rifle commenced in the late 1970s. Its prototype was based on the previous SG 540 design. In 1984 its designation was changed to the SG 550. Its production commenced in 1986. The SIG SG 550 was officially adopted by the Swiss army in 1990 as the Stgw.90. It replaced the ageing SIG 510 (Stgw.57) automatic rifle. Swiss army took the last deliveries of the Stgw.90 in the mid 1990s, however these weapons are still manufactured for export customers. Over 600 000 of these assault rifles were produced. Currently it is a standard Swiss military rifle. The SG 550 is in service with more than 20 operators worldwide. These are usually used by elite and special forces units.
It is a gas operated, selective fire weapon, chambered for a standard NATO 5.56×45 mm ammunition. Its operation system is similar to that of the AK-47. It is worth mentioning that prototypes of the SG 550 assault rifle were tested with 5.6×48 mm and 6.5×48 mm ammunition, however these cartridges were rejected and the standard NATO round was selected. Even though the 6.5 mm versions was more accurate than a standard NATO 5.56 mm version. The SIG SG 550 is one of the finest 5.56 mm assault rifle ever made. This weapon also proved to be reliable. Overall this weapon combines accuracy of the M16 and reliability of the AK-47.
A safety / fire mode selector switch is located on the left side of the receiver. It has semi-auto and full-auto settings. An additional optional trigger module also has a three round burst setting.
Swiss army SG 550 assault rifles are usually fed from 20-round translucent magazines. However 5-, 10-, and 30-round capacity magazines are also available. Standard magazines can be clamped together for quicker reloading.
Sighting equipment consists of a flip-up front sight and adjustable rear diopter. It has a sighting range of 400 m. Every rifle can be fitted with a detachable scope mount. Swiss army rifles are often used with 4x magnification scope. Current production models are available with a Picatinny-type rail and can mount various scopes, red dot, or night vision sights.
The SG 550 comes with a skeletonized side-folding buttstock. One interesting feature of this assault rifle is that it can be fitted with detachable folding bipod, which folds to the handguard when not in use.
The standard SIG SG 550 is compatible with bayonet and is capable of firing riffle grenades. This assault rifle can be equipped with the GL5040 40 mm underbarrel grenade launcher. This grenade launcher can be operated independently from the rifle. Some newer models are available with Picatinny-type accessory rails, mounted on the foregrip.