Swiss SIG SG 550 assault rifle
Birth history—
In the late 1970s, the Swiss Army planned to develop a new small-caliber rifle to replace the outdated Stgw57. The ammunition used in the new gun is selected between 5.6× 48mm Igel and 6.5× 48mm GP 80. There are two kinds of prototype guns, which are SG54 1 improved by S 1G Company on the basis of SG540 and SG42 of the state-owned Berne Arsenal.
Later, the Swiss army chose SG company's plan. Because NATO's 5.56×45 mm is adopted as the standard ammunition, it is required to improve SG54 1. 1983, the improved SG54 1 was officially adopted by the Swiss Army, and its shape was Type 90 assault rifle, referred to as Stgw90 for short. However, due to financial reasons, the Type 90 assault rifle was not put into production until 1986.
Main structure—
The structure of SG550 series assault rifles is similar to FNC rifles and AK-47 rifles. The automatic mode of long-stroke air guide piston is adopted, the piston rod is connected with the frame, and the gun head has two large locking ears. But the difference is that the compound spring is wound on the piston rod and located above the barrel.
The piston head has automatic closing function. When a small amount of gunpowder gas enters the air guide hoop to push the piston, so that the frame will recoil, the piston head will temporarily close the air guide hole and reduce the gas entering the air guide pipe. Moreover, there are air holes in the airway to discharge excess gas, so the amount of gas in the airway is limited to a small state to avoid the violent movement of moving parts.
In addition, there is a gas regulator on the gas guide hoop, which has two gas guide hoops with different sizes and a closing device. The muzzle is equipped with a "birdcage" flame arrester with a diameter of 22 mm, in which the muzzle flame arrester of SG550 can directly fire NATO standard grenades, and can also correspond to the SIG GL5040 gun-mounted Grenade launcher produced by Swiss Arms Company.
The detachable tripod can be installed on the lower guard wood of SG550, and can be folded in the tripod receiving groove of the guard wood when not in use, and the polymer gunstock can be folded on the right side of the chassis. Magazines are made of translucent plastic. The SG550 rifle equipped by the Swiss Army only distributes 20 magazines, but S 1G also produces magazines with the capacity of 5, 10, 20 and 30 magazines. Usually the SG550 standard used by overseas users will adopt 30 magazines. The barrel and piston rod of SG552 are very short, and the compound spring moves to the upper part of the frame behind the shell.
Although the setting of reentry spring in this position may be affected by the extreme working temperature of air guide piston and piston barrel, thus shortening the service life of the spring, the experiment of Swiss army proves that this design is beneficial to improve the shooting accuracy during repeated shooting.