Operating mechanism
The SG 540 is an air-cooled, gas-operated, selective fire assault rifle that fires from a closed bolt position. SIG designers used a long-stroke piston-driven action (with the recoil spring wrapped around the piston rod itself) and an adjustable gas regulator with three settings: "0"—used to launch rifle grenades, "1"—used in normal operating conditions and "2"—used in extreme environmental conditions or in the presence of heavy fouling. The firearm is locked into battery by means of a rotating bolt which features two heavy steel locking lugs. The bolt head is rotated by a control cam riding in a helical guide slot machined into the bolt carrier. When a shot is fired, propellant gases from the barrel are fed into the gas cylinder located above the barrel and impart rearward pressure on the piston rod, which drives the bolt carrier back. After a certain amount of free travel built into the system to allow the projectile to depart the barrel, residual gas pressures in the bore drop to a safe level, allowing the breech to be opened. The cam slot in the bolt carrier proceeds to actuate the camming stud in the bolt, rotating the bolt head out of engagement with the locking abutments in the receiver. The spent cartridge casing is extracted by a claw extractor contained in the bolt assembly while a protrusion on one of the internal bolt carrier guide rails in the upper receiver acts as the fixed ejector, removing the casing through a port on the right side of the receiver housing.
Features
The rifle is hammer-fired and has a fire control selector mechanism that is also the safety (lever in the "S" position—indicates the gun is safe, "1"—semi-automatic fire, "3"—3-round burst and "20"—fully automatic fire). The SG 540 is fed from either 20 or 30-round staggered box magazines. After the last round is fired, a hold-open tab in the magazine engages the bolt catch that locks the bolt in its rear (open) position. A bolt release button is located on the left side of the receiver.
The rifled barrel is fitted with a flash suppressor, which is also used to attach rifle grenades and a bayonet. The iron sights consist of a hooded foresight that can be corrected in elevation and an adjustable (windage only), rotating rear drum that contains four apertures with settings for firing at: 100, 300, 400 and 500 m. The receiver top surface can be used to mount adapters enabling the use of optics.
The SG 540 series come equipped with either a fixed buttstock or a side-folding metal skeleton stock. Another standard feature is a folding bipod which is stowed under the handguard when not in use. The fixed stock, pistol grip and handguard are all made of high-strength polymers to reduce overall weight. In order to minimize costs, metal stamping was employed in the production process.
SIG SG 540 Variants
Differences between the SG 542 and the SG 540 are mainly the result of adopting a more powerful, full-size rifle cartridge. The SG 542 also uses normally 20/30-round box magazines (5 and 10 magazines were also manufactured) but the sights were modified with a 600 m setting in addition to the 100, 300 and 500 m sight drum positions.
The SG 543 carbine is a shortened variant of the SG 540 that has been modified with a short 300 mm (11.8 in) barrel, a shorter gas cylinder and piston rod as well as a different handguard and sights with 100, 200, 300 and 400 m firing range positions. The SG 543 lacks the integral bipod of the SG 540/542 and the reduced barrel cannot be used to fire rifle grenades. The SG 540 served as the basis for the SG 541 prototype, which would later become the SG 550 that entered service with the Swiss Armed Forces as the Stgw 90 (Sturmgewehr 90).
Additionally, two semi-automatic variants were manufactured by MANURHIN between the late 1970s and the early 1980s specifically for the European civilian market: called the SIG Manurhin F.S.A. and the SIG Manurhin C.S.A.. These rifles were chambered in .222 Remington and .243 Winchester respectively. Both were available with either 30-round or reduced 5-round magazines, and the option of a folding or fixed stock. The unusual chambering for this type of military-style rifle was an attempt to comply with regulations in certain European countries imposing restrictions on the civilian ownership of "military" cartridges such as the 5.56x45mm NATO/.223 Remington or 7.62x51mm NATO/.308 Winchester.
| SG 540 | |
|---|---|
| Type | Assault rifle (SG-540, SG-543) Battle Rifle (SG-542) |
| Place of origin |  Switzerland |
| Service history | |
| Used by | Burkina Faso, Cameroon, Chad, Chile, Congo, Cote d'Ivoire, Djibouit, Ecuador, France, Gabon, Lebanon, Mauritius, Nicaragua, Nigeria, Oman, Paraguay, Portugal, Senegal, Seychelles, Swaziland, Togo |
| Production history | |
| Designer | Schweizerische Industrie Gesellschaft (SIG) |
| Designed | 1970s |
| Manufacturer | MANURHIN INDEP FAMAE |
| Produced | 1977–2002 |
| Variants | SG 542, SG 543 |
| Specifications | |
| Weight | SG 540: 3.52 kg (7.76 lb) fixed stock SG 542: 3.55 kg (7.8 lb) SG 543: 3 kg (6.6 lb) folding stock |
| Length | SG 540: 950 mm (37.4 in) stock extended / 733 mm (28.9 in) stock folded SG 542: 1,000 mm (39.4 in) stock extended / 754 mm (29.7 in) stock folded SG 543: 805 mm (31.7 in) stock extended / 569 mm (22.4 in) stock folded |
| Barrel length | SG 540: 460 mm (18.1 in) SG 542: 465 mm (18.3 in) SG 543: 300 mm (11.8 in) |
| | |
| Cartridge | SG 540, SG 543: 5.56x45mm NATO SG 542: 7.62x51mm NATO |
| Action | Gas-operated, rotating bolt |
| Rate of fire | 650–800 rounds/min |
| Muzzle velocity | SG 540: 980 m/s (3,215 ft/s) SG 542: 820 m/s (2,690.3 ft/s) SG 543: 875 m/s (2,870.7 ft/s) |
| Effective range | 100–500 m sight adjustments |
| Feed system | 20, 30-round detachable box magazine |
| Sights | Rear: rotating diopter drum; front: hooded  post |
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