SG 540

The SG 540 was developed in the early 1970s by Schweizerische Industrie Gesellschaft (SIG) of Neuhausen, Switzerland, today SAN Swiss Arms AG. It was a private venture aimed at export markets and at replacing the SG 510 (Stgw 57) in Swiss service. Work on a lightweight rifle firing an intermediate cartridge began at SIG during the 1960s and produced the SG 530-1, whose gas-assisted roller-delayed blowback mechanism proved too complex and costly to manufacture. In 1969 SIG abandoned the roller-delayed action in favour of a gas piston and rotating bolt system derived from the AK-47. The design entered production as a family comprising the SG 540, the SG 542 battle rifle and the SG 543 carbine. Production began between 1973 and 1974, but Swiss federal export restrictions meant manufacture was carried out at the Manurhin (Manufacture de Machines du Haut Rhin) plant in Mulhouse, France.

The rifle is air-cooled, gas-operated and selective fire, firing from a closed bolt. It uses a long-stroke piston action with the recoil spring wrapped around the piston rod, and an adjustable gas regulator with three settings: "0" for launching rifle grenades, "1" for normal conditions and "2" for extreme environments or heavy fouling. Locking is by a rotating bolt with two steel lugs, rotated by a control cam riding in a helical slot in the bolt carrier. Extraction is by a claw extractor in the bolt, while a protrusion on an internal bolt carrier guide rail acts as a fixed ejector, expelling cases through a port on the right side of the receiver. The weapon is hammer-fired, with a combined selector and safety marked "S", "1", "3" and "20". It feeds from staggered box magazines; a hold-open tab in the magazine engages the bolt catch after the last round, with a bolt release button on the left side of the receiver. The barrel carries a flash suppressor that also accepts 22 mm spigot rifle grenades and a bayonet. Iron sights comprise a hooded, elevation- and windage-adjustable foresight and a rotating rear diopter drum with four apertures. The receiver top surface accepts adapters for optics. Stocks are either fixed or side-folding metal skeleton types, and a folding aluminium bipod stows in grooves in the lower handguard. Fixed stock, pistol grip and handguard are high-strength polymer, and the design makes wide use of pressed and stamped steel components in place of machined parts to reduce cost.

The SG 542 differs mainly through its chambering, with a sight drum adding a 600 m setting. The SG 543 carbine has a shortened barrel, shorter gas cylinder and piston rod, a different handguard and a sight drum graduated to 400 m; it lacks the bipod and cannot fire rifle grenades. The SG 540 formed the basis of the SG 541 prototype, which became the SG 550, adopted by the Swiss Armed Forces as the Stgw 90. Manurhin produced two semi-automatic civilian variants between the late 1970s and early 1980s, the SIG Manurhin F.S.A. in .222 Remington and the C.S.A. in .243 Winchester, chamberings chosen to comply with European restrictions on civilian ownership of military cartridges.

The series entered service in Africa, Asia and South America, with users including Bolivia, Chad, Chile, Côte d'Ivoire, Ecuador, Gabon, Lebanon, Nigeria, Oman, Paraguay and Senegal, along with police and security services such as the Lethbridge Police Service in Canada and the Argentine federal penitentiary service. France used the rifle in limited numbers during the late 1970s with the armed forces, RAID and National Police Intervention Groups before the FAMAS replaced it in the early 1980s. INDEP of Portugal bought licence rights for the SG 543 in 1988; Portuguese Commandos and the Special Operations Troops Centre used the SG 543-1 until 2019, when the FN SCAR replaced it. FAMAE builds the SG 540, SG 542 and SG 543 under licence in Chile, where they are being replaced by locally produced IWI Galil ACE rifles. FAMAE also derived the 9×19mm SAF submachine gun from the SG 540.