5.56mm SIG SG-540 assault rifle, with fixed butt and optional bipod

5.56mm SIG-Manurhin SG-540 assault rifle as made in France under license, with side-folding butt


7.62mm SIG SG-542 assault rifle, with fixed butt

5.56mm SIG SG-543 short assault rifle, with side-folding butt

	SIG SG 540	SIG SG 542	SIG SG 543
Caliber	5.56x45mm (.223 Rem)	7.62x51mm (.308 Win)	5.56x45mm (.223 Rem)
Length	950 mm	1000 mm	805 / 569 mm
Barrel length	460 mm	465 mm	300 mm
Weight empty	3.26 kg	3.55 kg	3.0 kg
Magazine capacity	20 or 30 rounds	20 or 30 rounds	20 or 30 rounds
Rate of fire	650-800 rounds/min	650-800 rounds/min	650-800 rounds/min

The SIG SG-540 had been developed by the Swiss company SIG in the mid-1970s as a new,lightweight assault rifle, chambered in then-new small-caliber ammunition (.223 Rem / 5.56mm NATO). The SG-540 was licensed to the Manurhin of France and FAMAE of Chile, and had been temporarily used by French Army (who replaced it with FA MAS rifle in the early 1980s), Portuguese Army and Chilean army, and some other countries, mostly in South America and Africa. 7.62mm NATO version also had been developed, but found no significant sales. The SG-540 itself had not been adopted by Swiss military, but it became the platform for further improvements, which lead to the SIG-550 / Stgw.90 assault rifle. SG-540 currently is in production only by FAMAE company of Chile.
Early SIG prototypes, chambered in 5.56mm ammunition and known as SIG SG-530, were using some forms of roller locking, and thus, were too complicated and expensive to make. To made the rifle as cheap and reliable as possible, SIG designers selected the AK-47 style action, gas operated, with gas piston attached to the bolt carrier, and with rotating bolt with two massive lugs. The recoil spring is located around the gas piston rod, and the bolt carrier attached to the gas piston rod by the removable charging handle. The gas port has gas regulator with two different open and one closed position (latter for firing rifle grenades). The receiver is made from stamped steel and has two major parts, upper and lower, which are connected by pushpins. The barrel is screwed into the upper receiver. The trigger unit has a safety/fire selector switch on the left side of the receiver, with 3 settings: safe, semi-auto, full-auto. If desired, additional module could be installed in the trigger mechanism to allow 3-rounds burst mode. Rear sights are drum-type (like those found on Heckler-Koch rifles). SG-540 has muzzle compensator/flash hider of NATO-standard diameter, so it is possible to launch rifle grenades from the muzzle. The SG-540 has integral folding bipods under the handguard, and can be issued with fixed plastic buttstock or with side-folding tubular metallic buttstock.
The carbine version of the SG-540 is called SG-543 and has shorter barrel. SG-543 can't fire rifle grenades. 7.62mm version of SG-540 is known as SG-542 and visually differs mostly by rectangular magazine of greater depth. SG-542 is chambered for 7.62x51mm NATO ammunition. The civilian version of the SG-540 can fire only in-semi-auto and can be chambered also for .222 Remington cartridge. Other chamberings such as 7.62x39mm Russian (for SG 540) and .243 Winchester (for SG 542) are possible but extremely rare.