How does a rifle grenade work

Table of Contents

The SG550 / Stgw 90 is a light assault rifle and the standard weapon of the Swiss Army. To date, around 450,000 assault rifles have been delivered to the Swiss Army. The civil version SG550 PE is also very popular with sport shooters and has been sold over 33,000 times in Switzerland alone. In addition to Switzerland, versions from the SG500 line are in military or police use in Germany, France, Chile, Indonesia, Spain, Brazil and the USA. The Swiss Guard is also armed with the weapon.

At the beginning of the 1980s, the Swiss Army was looking for a replacement for the Assault Rifle 57.

In 1981 the SIG 550 was tested in a troop test. It was preceded by numerous predecessors, namely the SIG 530, 540, 542 and 543. After numerous endurance tests, it was officially introduced in 1990. The rifle is manufactured today by SAN Swiss Arms AG, the successor to SIG Arms AG. The name SIG 550 changed with the change to SG 550.


The SG550 model line works with indirect gas pressure and a screw cap system. This functional principle guarantees optimal functionality even under extreme conditions. Tests in the alpine regions of Switzerland showed the high level of precision and functional reliability of these rifles.


The SG550 is the standard, regular size model. Introduced into the Swiss Army under Assault Rifle 90, Stgw 90 for short. In French barrel 90 (Fusil d'assaut 90).

The sniper version is no longer in production. It was one of the few sniper rifles in caliber 5.56 mm.

The SG551 is the shortened version, which is used in the GSG 9, among others. It is particularly suitable for use in the maritime environment. The SG551 Long Barrel is optimized for firing rifle grenades, like the SG550, a bayonet can be mounted.

With the dimensions of a submachine gun, the SG552 is the most compact version. It was introduced as a corps weapon in the Swiss Army. Due to the shortened gas rod, it is the only model with the closing spring Behind the closure. This must be held when opening.

All versions are optionally available with a diopter sight or Picatinny rail as well as civil versions (PE) according to the Swiss Weapons Act. Optionally, a Picatinny rail with a screw connection can be clamped into the models with a visor.

Overview - technical data

descriptionlength Gun weight Muzzle energy
SG 550 100cm / 77cm 4.1 kg 1700 joules
SIG 550-1 sniper 113cm / 91cm 7 kg n / a
SG 551 SWAT 83cm / 60cm 3.4 kg 1460 joules
SG 551 long barrel 92cm / 70cm 3.6 kg 1600 joules
SG 552 Commando 73cm / 50cm 3.2 kg 1100 joules



The Swiss Army uses the rifle cartridge 90, or GP 90 for short, in caliber 5.56x45mm (= 0.223 inches), rounded up by the Army to 5.6 mm [1]. It differs only minimally from the M193 nuclear ammunition, which weighs 62 grains in contrast to the GP90 with 63 grains[1]. For the highest possible precision, however, two different barrels with twist lengths of 178 mm (7 ") and 254 mm (10") are available for the two types of ammunition.


In contrast to most assault rifles, the SG550 does not work with a direct trigger, but with a pressure point trigger, which favors precision shooting. Due to the low manufacturing tolerances, the forged, cold hammered and hardened barrel and the GP90 ammunition, the SG550 is one of the most precise assault rifles in the world. All rifles that leave the factory are tested for precision in the manufacturer's underground 300m shooting range in Schaffhausen, Switzerland. The scattering circles with normal army ammunition (GP90) must not exceed a diameter of 10 cm in a series of 10, otherwise the rifle must be reworked. Accordingly, in principle every assault rifle in the Swiss Army can meet this requirement.

Fire choice

In addition to single fire and fully automatic, the SG550 line also has a short fire setting, which fires three shots when the trigger is pulled. The positions are marked as 1, 3 and 20.


In general, the models of the SG550 line are very manageable, not front-heavy and easy to grip. However, large hands only have barely enough space on the pistol grip, but the rounded end makes this easy. A special feature is the shaft, which can be folded in in tight spaces, such as in house-to-house warfare, without restricting its function.


Bursts of fire can be controlled relatively well with all models of the SG550 line; the rifle does not have a tendency to break away in any particular direction. There is only a certain amount of throw up, but it is easy to control, even with the very short, lighter SG552.


The magazines are made of impact-resistant, transparent polymer plastic and are 30% lighter than magazines made of metal. The charge status is thus visible. For quick magazine changes, the 20 and 30 round magazines can be coupled practically without limits (more than 3 pieces are not practical); 5 and 10 round magazines are also available.


Positions 1, 2, 3 and 4 can be set on the sight drum. They correspond to the shooting distances of 100 m, 200 m, 300 m and 400 m. The sight position 1 is designed as an escape sight, and two light cells are attached to the side, which serve as night sights. A hinged night front sight with a light cell complements the night sight.

Gas nozzle

With the SG550 line, the amount of gas required to ensure proper functioning can be controlled by switching the gas nozzle from the smaller to the larger through-hole. The gas nozzle is located at the front end of the gas cylinder under the front sight and can be adjusted by hand. Basically, shooting is done in the "vertical" position in order not to unnecessarily strain the rifle and not to increase the recoil. If it is very dirty or iced up, the gas nozzle is moved to the second position in order to prevent feed or ejection problems. In the basic state of the rifle, changing the gas nozzle increases the rate of fire to up to 1,000 rounds / min, but conversely it can also cause feed or ejection problems.


The SG 550 is an indirect gas pressure charger, the structure is almost identical to an AK 47. The gas pressure required for automatic reloading is not transmitted directly to the lock, but via a joint. This means that contamination from burned-off powder residues cannot penetrate further than the gas pipe.

In a series of tests by the Swiss Army, the Stgw 90 was tested under combat conditions. After every five 20 magazines, breaks were taken to let the barrel cool down; around 15,000 rounds were fired with a single rifle without cleaning the rifle in between. There were no jams, material fatigue or loss of precision.

The fact that the SG 550 is often said to be susceptible to jamming could be due to the fact that in civilian shooting ranges the fired, sometimes slightly damaged cartridge cases are often reloaded, as well as the plastic case deflector used, which in the firing range is due to the very strong case ejection onto the rifle has to be attached and partially directs the ejected sleeves randomly directly back into the closure, which then jams the sleeve.

GL 40 grenade launcher

The GLG 40 is a single loader for 40 mm grenades. It can be attached to the SG550 (GL 5040) and SG551 (GL 5140) by exchanging the lower hand guard. All available grenades in caliber 40 x 46 mm with a cartridge length of 100–198 millimeters can be used.

The grenade launcher is also suitable for police operations as a primary intervention weapon. It weighs 2.5 kg without the grenade, and its operational range is between 25 and 200 meters.


The Picatinny 4-rail handguard milled from aluminum enables the simultaneous installation of optical aiming aids / night vision devices and tactical lights such as laser and white light. The lower rail allows a vertical handle and / or a bipod to be mounted at the front. The Picatinny handguard can be attached or removed in seconds without tools and is available for all versions.


Civil nameSIG 550
Military designationAssault rifle 90 (Stgw 90)
Country of operationSwitzerland, Germany, France, Chile, Indonesion, Spain, Brazil, USA, Vatican
Developer / manufacturerSIG Holding / Swiss Arms
Country of manufactureSwitzerland
Model versionsSIG 550, SIG 550-1 Sniper, SIG 551 SWAT, SIG 551 Long Barrel, SIG 552
Weapon categoryArmy weapon / assault rifle



Case and propellant charge