SIG SG 540 The SG 540 is a 5 . 56mm assault rifle developed in the early 1970s by Schweizerische Industrie Gesellschaft ( SIG , currently SAN Swiss Arms AG ) based out of Neuhausen , Switzerland as a private venture primarily destined for export markets and as a potential replacement for the 7 . 5mm SG 510 automatic rifle known as the Stgw 57 in Swiss service . History . Work on a new generation of lightweight rifle using the intermediate 5 . 56mm round began at SIG in the 1960s and resulted in the somewhat unsuccessful SG 530 - 1 design that employed a gas - assisted roller - delayed blowback operating mechanism which proved too complicated and expensive to manufacture . As a result , in 1969 SIG chose to dispense with the complex roller - delayed action opting instead for a design using the more robust and simple piston - operated rotating bolt locking mechanism ( derived from the Soviet 7 . 62mm AK assault rifle ) . This combination was to be successful and the rifle entered production as a family of weapons consisting of the base 5 . 56mm SG 540 , the 7 . 62mm SG 542 battle rifle and the 5 . 56mm SG 543 carbine , where both 5 . 56mm variants were designed for use with 5 . 56x45mm NATO M193 ammunition , while the SG 542 — the full size 7 . 62x51mm NATO rifle cartridge . Production began between 1973 – 1974 , but due to Swiss federal restrictions on firearms exportation , manufacturing had to be carried out at the MANURHIN ( " Manufacture de Machines du Haut Rhin " ) facility in Mulhouse , France . The SG 540 series has entered service with the armed forces of several countries in Africa , Asia and South America as well as numerous law enforcement and security services . In 1988 , the nationally - owned Portuguese defense manufacturer INDEP purchased rights to license build the SG 543 carbine for its own forces . Currently both the SG 540 and SG 542 are only manufactured in Chile , under license by FAMAE for the Chilean Armed Forces . Based on the SG 540 , FAMAE developed a 9mm submachine gun version of the weapon known as the SAF . Design details . 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 grenade s , " 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 grenade s and a bayonet . The iron sight s 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 . 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 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 .