Staff Writer (Updated: 8/9/2016):
The SG 550 was developed into a family of automatic weapons, each born from the basic automatic action and intended to be inherently modular to produce specialized carbine, commando and sniper rifle forms. This benefitted prospective buyers by way of its logistically friendly design that could arm frontline, secondline and specialized forces units as needed - all sharing the same internal components, ammunition and function/training.
The SG 550 was designed as an "soldier's weapon" and special care was taken in produce the most lightweight and inexpensive rifle yet one that was utterly reliable and worthy of frontline service. The SG 550 therefore incorporated plastics alongside its metal construction to produce an overall lighter assault weapon. Only key components - those required to absorb the punishment of the firing action - were left metal while the shoulder stock, handguard and magazine were all of plastic. Standard tritium iron sights were fitted for low-level light work though optics could be affixed across the top of the receiver as required. The weapon utilized a gas-operated, rotating bolt action in which the gas cylinder was mounted over the barrel and both of the assemblies were shrouded over by the plastic handguard. The handguard also integrated a folding bipod for the squad support/suppression role. The shoulder stock was skeletal (another weight-saving measure) and designed to fold over the side of the receiver without impeding the functionality of the rifle. Controls of the weapon were fully ambidextrous and included a firing selector with a safety, single-shot, three-round burst and full-automatic mode. The magazines - in 5-, 20- or 30-round counts - were constructed of transparent plastic to allow the operator to receive a quick visual read-out of his existing ammunition supply. Each magazine was also designed with slots to allow several magazines to be clipped together (all standing vertically) for an increased ammunition supply - the operator simply ejecting the spent magazine and sliding the next available magazine into the well. The magazine release lever was situated between the trigger guard and magazine well in the usual way and available mountings allowed use of a bayonet. The muzzle was designed as such that it could fire rifle grenades to further broaden the tactical reach of the standard infantryman.
Despite its extremely promising design, the SG 550 was slow to arrive into Swiss Army service. Budget concerns invariably delayed the arrival of the weapon system to frontline units for several years as funding was diverted to more critical components. Serial production, therefore, did not begin until 1986 and it was only in 1990 that the SG 550 began formal service with the Swiss Army in full. Production of non-military SG 550s is ingoing as of this writing (2012) with some 600,000 examples of all types having been produced to date.
The SG 550 series has been branched out beyond its original full-length assault rifle form to include specialized variants for the discerning customer. Each was more or less faithful in their design to the original SG 550 including internal working components and overall function. The SG 551 became a shortened pattern carbine form of the full-sized 550 complete with a 14-inch barrel and applicably shortened gas cylinder and pistol. This version did, however, lose the capability to fire rifle grenades, mount a bayonet and utilize a folding bipod for obvious reasons. The SG 551-1P (P= "Police") was a police-minded version of the 551 while the SG 551 SWAT was a specialized police version. The SG 551 LB (LP = "Long Barrel") was yet another carbine derivative though with an 18-inch barrel which retained the capability to fire rifle grenades and mount a bayonet. All carbine versions accepted a 20- or 30-round magazine.
In 1998, the SG 550 family was further developed into a more compact version as the SG 552 "Commando" with the police version being the SG 552-2P. This form included a 9-inch barrel and reworked internals due to its reduced overall size. There also existed the SG 552 LB long-barrel form with its 13.6 inch barrel assembly. In 2009, SIG released the improved compact SG 553 with 13.7 inch barrel, adjustable shoulder stock and Picatinny rail accessories support. Internals were reworked to match more closely that of the original SG 550 mark.
A dedicated sharpshooting platform was unveiled as the SG 550 "Sniper" with appropriate modifications for the accuracy-minded, long-range role. This variant included a longer barrel of heavy-duty construction, an adjustable butt with cheekpiece, a two-stage trigger, a well-molded pistol grip and support for a variety of optics. This mark lacked the basic iron sights as optics were expected to be a part of the complete weapon system for obvious reasons. Magazine counts included a 5-round and 20-round form.
The SG 550 was inevitably released to the commercial market and limited to semi-automatic fire forms. As such, they fit the bill well for civilian customers and police force, the latter generally not requiring use of full-automatic weapons. "Americanized" versions were seen with STANAG magazines and featured an M4 Carbine-style collapsible (telescoping) stock while a sporting-minded version - the SIG522LR - was chambered for the .22 cartridge.
The SIG 550 family enjoys widespread use in the army, police and security forces of the world. Operators include Argentina, Brazil, France (as the Fass 90), Germany, India, Indonesia, Macau, Malaysia, Malta, Poland (GROM), Serbia, Slovakia, Spain, Switzerland, Turkey and the United States (solely the DEA).