SIG SG 540
The SIG SG 540 was intended to replace the aging Stg 57, but by the time production got up to speed, the Swiss Army had jumped on the 5.56mm trend train and was looking for a rifle to fire that round. Production of the 540 began in 1977, and continued under contract until the early 1990's. A few 540's were made under contract by Manhurin in France for the French Army, chambered in 5.56mm, until FAMAS production got up to speed. These were known as SG 542s. For about ten years after SIG had stopped making the SG 540, they were made under contract in Chile for their army, until the early 1990's when the 550 series was adopted. Although they resemble the H&K G3, the similarity is only skin deep, literally. The SIG system is completely different from H&K's. The SG 540 departed from the roller delayed blowback system popularized by H&K, and endorsed a modified Kalashnakov action. The mechanism is actually quite simple. After ignition, a portion of propellant gasses are directed into the gas vent, and thus into the cylinder. They then drive the piston back, to which the bolt carrier is attached. After a short free travel to allow excess gas bleed off, the cam slot engages the bolt's cam pin, which rotates and unlocks the bolt as the carrier moves rearward. At this point, primary extraction occurs. As the casing is extracted as the bolt is rotated, so the huge Kalashnakov extractor is done away with. Ejection is violent, denting the casings severely as they are flung to the right and forward. At this point, the recoil spring is compressed, and its return energy drives the bolt carrier forward, stripping a fresh round from the magazine and chambering it. These rifles are extremely rare, and in fact, I have only ever seen one for sale on the civilian market, as no civilian model was ever produced. The open sights are diopter click, and the front sight is a hooded post. As seen in the photograph, these rifles can be fitted easily with telescopic sights. A few models were also made in .222, .243, and 7.62 x 39.
Picture from Kyle Giffen