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”. 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).
In February 1983, the Swiss Federal Council made public its decision that the new 5.6mm SIG SIG541 assault rifle (renamed the SIG550 at the end of October 1984) would be adopted as the standard service weapon of the Swiss Army. About 600,000 weapons would be produced, with the cost of manufacturing the rifle and its ammunition estimated at SFr1,600 million, to be spread over 15 years. The rifle will enter service at the beginning of the next decade under the Swiss Army designation of Stgw90, replacing the 7.5mm Stgw57. In autumn 1983, however, a credit of SFr85 million was voted to enable SIG to manufacture a first series- production batch of 15,000 rifles for delivery in 1986. Two thousand of these should be ready by the end of 1986 for handing over to the army for extensive in-service evaluation during which the rifle could undergo minor modifications to improve its handling. According to the timetable already established, SIG will receive the principal order for 135,000 rifles in 1987, for delivery beginning in 1990. As was the case with the Stgw57, a third of the total production work will be go to the Federal Arms Factory in Berne, which will carry out the final assembly of 80% of the weapons, while another third will be shared out among other Swiss companies.

The genesis of the Stgw90

In the second half of the 1970s, the Swiss ministry of defense undertook to find a replacement for the Stgw57. After a series of in-service tests, carried out in 1978, of 40 modified SIG SG540 and SG543 rifles, the Gruppe fur Rustungsdienste (GRD) drew up a staff requirement. In addition to the usual specifications concerning handling, ruggedness and safety of operation, the new rifle had particularly to meet the following requirements:

1 – That it should serve as the basis for a family of weapons, including a standard rifle and a shortened version, the latter to be used as a personal weapon for headquarters staff, commanders, tank crews, paratroops, and special units.

2 – That it should be at least as ac curate as the Stgw57 at 300m, this range being specified because of the weapon I s widespread use at this distance in nonservice target-shooting in Switzerland ‘

3 – That it should be much lighter than the Stgw57 which is considered to be too heavy and bulky, thus restricting the soldier’s battlefield mobility.

After having ruled out the West German G1 I rifle, which uses caseless ammunition, the GIRD carried out a competitive evaluation of two weapons: the C42 developed by the Federal Arms Factory and SIG’s SG541, or, as it is now, the SG550.

General characteristics

The SG550 stripped. From left to right and from bottom to top: gas regulator, upper handguard, barrel/receiver assembly, cocking lever bolt carrier, bolt and firing-pin retaining pin, firing pin and spring, lower handguard, lower-receiver take-down pins, butt/trigger/lower-receiver assembly ‘ hinged bipod, magazine base, box magazine magazine spring and follower, charger, bayonet and scabbard, cleaning kit, and sling.

The SG550 is mechanically similar to the SG540 and SG543 rifles already described in the IDR (7/1982, p.875) and which were used as a base for perfecting the new rifle. SIG has again designed a gas-operated weapon with a piston acting on the bolt carrier, which locks and unlocks a rotating bolt with two front lugs. That apart, the SG550 is much less complex than the Stgw57, made up of only 174 parts, compared to the 237 of the weapon it will replace. The use of sheetmetal stampings and plastic material has permitted the weight to be reduced to 4kg for the standard rifle fitted with an empty magazine, 0.1 kg less than the maximum weight stipulated by the GIRD. Armed with the rifle and the standard service issue of 120 cartridges, the Swiss soldier will not have to carry more than 6kg, a load lighter by 4.3kg compared to the Stgw57 with the same amount of ammunition.

In accordance with the staff requirement, the new service rifle is being offered in two versions: the SG550 standard weapon, measuring 100cm in length, and the SG551 short-barrel headquarters weapon which is only 82.7cm long and weighs 3.43kg. Both are fitted with a stock which can be folded, bringing their respective lengths down to 77 and 60cm. Unlike the standard version, the SG551 has no bipod or bayonet attachment lug and cannot fire rifle grenades.

Although the general design of the new rifle owes a good deal to that of the SG540, SIG has introduced numerous improvements which make the SG550 one of the most carefully conceived assault rifles of the present generation. A description of the gas operation is the best way to support this statement. Instead of entering the gas cylinder directly, as is the case with many other gas-operated weapons, the gas passes through the port of the gas intake into an Lshaped channel machined in the piston head. It is thus directed forward towards the cylinder head/gas regulator and the build-up of pressure over the piston head makes the piston recoil, transfering its energy to the bolt carrier, which in turn unlocks the bolt. As the piston moves back however, the gas port and the L shaped channel move out of alignment and the piston head thus blocks the port, cutting off the supply of gas to the cylinder. The latter is fitted with an exhaust port through which the surplus gas in the cylinder and the combustion residues are evacuated. This system ensures that only the exact amount of gas needed to impart the necessary momentum to the moving parts is admitted from the bore. The moving parts are therefore subjected to less mechanical stress, increasing their useful life. The rifle is fitted with a two-position gas regulator, which is adjusted either by hand or, if necessary, with the aid of a cartridge or bayonet, so that if the weapon becomes seriously fouled the supply of gas can be increased. With the same aim of reducing wear on the most used parts, the interior of the receiver is fitted with rails which guide the bolt carrier and bolt, thus reducing the transversal stress.

Accuracy – a fundamental requirement
A close-up revealing the ingenious simplicity of the SG550. The single forward ring holds the gas intake, the gas cylinder and its regulator, the front sights, the bayonet lug and the sling swivel The gas regulator has two positions one for normal operation and one in which the gas supply is increased, for use when the weapon is seriously fouled. The photo shows the night sight raised hiding the day sight. The protective foresight hood is also used for aiming – to give a range of 75m when launching 500g FN/Luchaire Type 58-N anti-tank grenades. These grenades can be fired using standard ammunition.

In order to obtain the firing accuracy required by the GRID, SIG has in particular modified the barrel – manufacturing process. The chamber is now forged at the same time as the bore so that irregularities in alignment which can occur when the bore is hollowed out after hammering are avoided.

The rear sight is soldered on to the back of the receiver. It is fitted with a fourposition drum adjusted by means of micrometer screws for elevation and windage. Adjustment of the drum by one notch displaces the point of impact by 25mm at 100m. At ranges of 200, 300 and 400m, the firer aims through an aperture sight. For firing at 100m or less, he has an open sight, which is flanked by two luminous tritium spots for night firing. The foresight is protected by a hood and is doubled by a folding night sight which, when erected, reveals the third luminous spot necessary for night firing. The sight base is 540mm (466mm on the SG551 ), which is one of the longest on a modern assault rifle after the Heckler & Koch G41 (556mm). When the day sight is adjusted, the night sight is also adjusted automatically.
The aluminium bipod is no longer fixed by a ring to the barrel but its attachment is slid inside the lower handguard and held in place by the upper handguard. It therefore no longer contributes to barrel vibration, a phenomenon which, in the case of the SG542, could displace the point of impact by 20cm at 300m.

The combination of these characteristics makes the prototype SG550s slightly more accurate than the Stgw57, as confirmed by the Swiss Army during comparative tests of 73 of the long version against Stgw57s. With the rifles held firmly in clamps, serials of 24 shots were fired at a range of 300m. The new rifle achieved an overall grouping of 50% of hits in a 7 x 6cm rectangle (226 points out of a possible 240), while the Stgw57 achieved a grouping of 7x8cm (220 points).

Even more significant was the result of a test in which 24 long-version rifles, mounted on their bipods, were used by four firers, each firer firing two serials of ten shots each from each rifle. The average grouping (100 points) was 21.8x I 9.4cm (91.1 points) when the aperture sight was used and 18.4x17cm (93.1 points) when a telescopic sight with x 4 magnification was used. When eight of the headquarters weapons were tested under the same conditions, it was found that the shorter barrel did not affect accuracy when the telescopic sight was used (rectangular grouping of 18.4×16.8cm – 93.6 points) although, with a sight base shorter by 74mm, there was some effect on accuracy when the mechanical sights were used (25 x 22.2cm. – 88.8 points).

Handling characteristics

The SG550 and SG551 are not far from perfection as far as their human engineering is concerned. Operation of the rifle is as easy for left-handed firers as it is for right-handed ones, as far as manipulation of the cocking handle (slightly turning the rifle if necessary), the magazine catch and the fire-selector on the right-hand side of the receiver are concerned. The fire-selector, which is combined with the manual safety, falls easily under the thumb and can be manipulated by the firer without his having to take his hand off the pistol grip. The boltstop catch is perhaps more difficult for the left-handed firer to manipulate, but he can free the bolt by a light pull on the cocking lever, which is covered with soft rubber. Finally, the trigger guard is hinged and can be swung to the left or to the right for firing with Arctic-gloved hands.

After the first in-service trials, SIG modified the shape and position of the pistol grip, both to obtain the optimum distance between the hand and the trigger, and to bring the hand nearer to the axis of the bore to increase stability when firing and to reduce muzzle climb. OR was able to confirm during an exclusive test accorded it by SIG that these aims have been achieved, both in semi-automatic and in automatic fire.

Because the sights are quite low, standing about 35mm high from the axis of the barrel, the firer, when in the prone position, presents a target which is about 20% smaller than is the case with the Stgw57. The relatively thin stock facilitates the instinctive shouldering of the rifle for firing, with the shooting eye falling naturally on to the line of sight. The stock is semi-skeleton in form, thus lightening the rifle, but above all offering a very firm grip if the soldier has to deal a blow, parry a thrust, or use his bayonet. The butt plate is made of hardened non-slip rubber which gives excellent grip even when the rifle is shouldered against a wet or snowcovered battledress, and which allows the weapon to be used as a support to help the soldier in mountainous terrain.

When the stock is folded along the right side of the rifle, it is held in place by a stud on the lower handguard which engages a stud socket in the butt. A good pull brings the stock back to its initial position into which it is locked automatically by a butt catch. It is to be regretted, however, that when the stock is folded it is relatively difficult for left-handed firers to remove or insert the magazine.
The thickness of the handguard has been kept down so that the soldier can comfortably carry his rifle by holding it with one hand near the chamber, under which is the weapon’s centre of gravity, thus obviating the need for a carrying handle.

The handguard extends as far forward as the front ring to prevent the firer’s hand from being burned by contact with an overheated barrel. This single ring fulfills several functions: it holds the handguard in place and it supports the gas intake port, the gas cylinder/regulator, the bayonet attachment lug, the sling swivel and the front sights and their protective hood.

Field-stripping and reassembly, which can be carried out without any tools, is simpler again than with the SG540, whether it involves withdrawing the cocking handle, extracting the gas cylinder, or dismantling the bipod and the handguard, the two sections of which slide into one another and are secured by hooks.

A well thought-out construction

The right-hand side of the SIG550, with the stock folded and held in place by a stud on the lower handguard. It should be noted that, in this position, the stock makes manipulation of the magazine awkward for left-handed firers. Two rubber sealing flanges seal the cockinglever channel and protect the mechanism from dirt. Ventilation slots in the handguard help to cool the barrel and raise the cook-off threshold to 240 rounds/min. The handguards are not bulky and the rifle can be carried in one hand, holding it level with the chamber, under which the weapon’s centre of gravity is situated.

While SIG has made no great innovations in the fabrication of the metal parts of the SG550 and SG551 compared with the SG540, the company has nevertheless increased the strength and rigidity of the receiver, particularly the magazine housing and the rear part against which the bolt assembly recoils. It has not been considered necessary to chrome the bore, but the barrel, which is drawn from a chrome- nickel -steel bar, is thicker, particularly at the chamber, where its diameter is 28mm. Great attention has been paid to protecting the exterior surfaces because of the severe conditions which can be encountered in Switzerland. After being phosphatised but before finishing, the metal is subjected to a treatment which smooths the surfaces and which, according to SIG, greatly increases its resistance to knocks and to corrosive agents.

The manufacturer seems to have made a technological breakthrough, however, in the use of a plastic material for the stock, handguard, pistol grip and magazine. Neither the composition of this material nor the identity of the Swiss laboratory which developed it has yet been revealed. Claimed to be far superior to Zytel in its strength and homogeneity, it is said to be capable of withstanding shocks, such as the firing of a grenade, at any temperature from -40’C to + 50 C without its properties being altered. The four surfaces of the handguard and the stock can be submitted to the pendulum test, against steel or concrete, without breakage or cracking resulting.

Despite the low weight of the magazine – 90g for the 20-round version, and 11 0g for the 30-round one – it is just as robust and, being transparent, permits a rapid check of the number of rounds remaining. Loaded, it will withstand being dropped from a height of 5m on to hard ground, even at -30’C. During endurance tests, it was thrown 45 times on to concrete from a height of 1.5m, with the lips facing downwards, without any deformation occurring. Even when empty, it can support the weight of a man without breaking or bending. Finally, it is f lameresistant, and in intense cold more comfortable to handle than a metal magazine. More than 800,000 rounds were fired using the first 40 of the 105 prototype weapons produced for the competitive evaluation phase, without any malfunctions attributable to the magazine. Since the reliability of an automatic weapon depends first and foremost on that of its magazine, such qualities deserve to be underlined, as IDR has already done in the case of the magazine used in the Steyr- Poch AUG-77 (see IDR 10/1982, pp.1 421 – 1424).

Drawing on the common practice of taping one or more magazines together (lying head-totoe in relation to another) to speed up reloading, SIG has taken out a patent for a “multipack” system, approved by the GRD which allows the soldier to fix three or more magazines under his rifle. To fit the magazines together, all that is necessary is to insert two studs on the right-hand wall of the second magazine into two U-shaped slots fitted against the lefthand wall of the first magazine which is engaged in the rifle. One slot is open towards the top and the other towards the front (see photo). A third magazine can be attached in the same way to the second. As all the magazines have slots and studs, there is no preferential order. One advantage of this system is that all the magazines are in the upright posi tion, lessening the risk of dirt entering the spare magazines if accidentally banged against the ground. Even if, despite all, they are exposed to sand, mud or snow, the presence of these foreign agents is more easily spotted because the magazines are transparent. In addition, the gap between each magazine has been carefully calculated so that the magazine can be switched with one hand without any fumbling – the author put this to the test at length.

In order to reduce the height of the weapon as far as possible, at the same time maintaining a gap between the magazine and the ground when the rifle is resting on its butt and bipod, the GRID has opted for the 20-round magazine. This decision is not surprising since Swiss doctrine has always laid greater emphasis on single, well aimed shots, and since automatic fire can be controlled by the three-round burstlimiter. SIG’s 30round magazine is therefore exclusively for export.

Ruggedness and reliability
SIG has taken out a patent for a system of juxtaposing several magazines under the rifle in order to speed up the reloading of the weapon. The right side of each magazine sports two studs which are engaged by a backward pivoting movement in U-shaped slots on the left-hand side of the next magazine. The 20round plastic magazine adopted by the Swiss Army a 30-round version is also available for export – is transparent and shock, flame and dirt-resistant. It weighs only 90g and is more comfortable to handle in intense cold than metal

The Swiss Army has not surprisingly laid down strict requirements concerning service life of the SG550 and its ability to function in severe conditions. SIG has taken full advantage of the 800,000 rounds fired during the first trials, and the pre-production prototypes successfully passed the GRD’s endurance tests which stipulated a minimum rifle life of 15,000 rounds – of which 7,500 rounds should be fired without failure (breakage) of a single part. Many rifles fired 20,000 rounds, some of them even 40,000, this despite the barrel being subjected to a rigorous cycle consisting of firing 100 rounds in a minute (20 on semi-automatic, 2 x 20 in three-round bursts and 2 x 20 on full automatic) followed by immersion in water, and by cleaning and inspecting for wear and accuracy every 1,000 rounds.

The cook-off threshold of the ammunition is high, being situated at 240 rounds/min compared with the 160 rounds/min specified by the GRD This results from the carefully calculated thickness of the rear end of the barrel which ensures good heat diffusion during tests, the highest temperature was recorded at a point 1 5cm from the chamber – and from the good ventilation given by the openings in the handguard. Operating safety is also increased by a safety catch which prevents the hammer from falling until the breech is completely locked. In accordance with the specifications, the rifle withstands the firing of at least 600 500g grenades with the stock braced against concrete (the manufacturer claims to have fired more than 1,000 without incident) and also the traditional 2m drop test, from any angle.
The SG550 and SG551 came through various reliability tests with excellent results, due in large part to the improvement made in waterproofing the receiver as compared to the SG540. Thus, the channel along which the cocking lever rides on each side is sealed by a flange of supple, resistant rubber which protects the mechanism from foreign bodies. The new rifles stood up well to exposure to dust in both static and dynamic tests which were followed each time by the firing of 100 rounds on semi-automatic and 2 x 20 rounds in bursts. They also coped successfully with the sand test: the loaded rifle, with the bolt locked, was dragged 20 times on each side through sand at an angle of 55 degrees over a distance of 2.5m and at a speed of 0.5m/s. After each time, the rifle had to fire five rounds without jamming. For the mud test, the GRD adopted the NATO norm of 12 successive baths. The SG550 and SG551 came through it all with flying colours.

The operational tests were carried out at a temperature of -20’C on the Jungfraujoch, at more than 4,000m (I 3,000ft) altitude, The tests included filling the loaded weapon, bolt open, with snow and leaving it until it was frozen solid. A soldier then had to be able to make it ready to fire within 30 seconds using only his service-issue equipment of knife, bayonet and cleaning kit … and by vigorous kicks on the cocking lever.

Equipment and accessories
The barrel is fitted with a flash suppressor with the standard diameter of 22mm. There are plans to design a new bayonet for the rifle even though the current service model can be fitted. Particular care has been taken with the shape and orientation of the flash-suppressor slots in order to reduce muzzle flash and avoid sand or snow being kicked up by the gas escaping once the bullet has cleared the barrel, when firing in the prone position.

Integral rails on the receiver, at the base of the chamber, and a conical mortise machined in the base of the back sight allow the fitting of day or night optical sights. This method has been borrowed from the SG540 where it has worked well. The Swiss Army maintains that a footsoldier should be able to fight tanks and therefore plans to use the FN/Luchaire 58N series rifle grenades which are attached to the flash suppressor and held in place by a collar. The tail of the grenade is fitted with a bullet trap so the projectile can be launched using normal GP90 ammunition. No sighting scale is necessary by lining up the hood protecting the front sight with the outer surface of the grenade, a range of 75m will be reached automatically. For other ranges, however, the firer himself has to estimate the correct angle of elevation.

The soldier will normally carry his spare ammunition in ten-round clips. Using a charger, he reloads the magazine by simple thumb pressure.

The service-issue cleaning kit, which was designed with the needs of competition shooting in mind, is quite voluminous and has to be carried separately. The manufacturer has, however, designed a smaller kit, which can be fitted in the rifle’s pistol grip, for export.

The standard version SIG550 and the short-version SIG551 head uarters weapon. Because of budgetary constraints, the Gruppe for R6stungsdienste (GRID) has provisionally shelved its plans to procure the SG551, even though this version fully meets the specifications of the staff requirement. SIG will nevertheless offer both versions for export. The short version is not fitted with a bipod and cannot be armed with a bayonet, nor can it fire grenades,

There is no doubt that SIG has succeeded in developing for the Swiss Army a small-calibre assault rifle which is a worthy successor to the Stgw57, despite the qualities which have earned for the latter a very flattering reputation. The SIG550 allows the infantryman greater mobility, gives him more firepower when firing on full automatic than was possible in practice with the 7.5mm calibre, and increases his efficiency by simplifying most of the operations required in the rifle’s use.

An important point which will certainly not have escaped the Swiss procurement authority’s notice is that the Stgw90 will cost about half the price of its predecessor without having to sacrifice any of the latter’s performance or operational qualities.

As far as exports are concerned, the new family of rifles will also be available with a 1 -in-1 78mm rifling twist to fire the SS109 NATO standard ammunition. They are not likely to be among the cheapest in their category, but these are comprehensively designed weapons. In addition to offering all of the functions a modern assault rifle is expected to perform, at a high quality/price ratio, they have an estimated service life of 30 years – which speaks volumes for their intrinsic sturdiness.

Calibre: 5.6mm (5.56x45mm)
Operation: Gas
Breech locking mechanism: Rotating bolt with two lugs

Rifle, stock extended: 1000mm
Rifle, stock folded: 770mm
Barrel: 528mm
Rifling twist: 450mm
Sight base: 540mm
Rifle, without magazine or bipod: 3.75kg
Rifle, with empty magazine and bipod: 4.0kg

Number of Grooves: 6
Cyclic Rate of Fire: 700-800rds minute
Muzzle Velocity (GP90): 920m/s
Muzzle Energy (GP90): 1700J

Mechanical Sights

1) four-position drum (100-400m), adjustable for elevation and windage
2) 100m night sight with folding foresight