The Sturmgewehr 57 is a selective fire battle rifle designed by Schweizerische Industrie Gesellschaft (now SAN Swiss Arms) of Switzerland. It uses a roller-delayed blowback system similar to the H&K G3 and CETME rifles. The AM 55 entered service in the Swiss Army with the designation F. ass. 57 7.5mm (French/Italian, for Fusil d'Assaut/Fucile d'Assalto 1957) or 7.5mm Stgw. 57 (German for Sturmgewehr 1957), with export variations constituting the SG 510 family of small arms. The Sturmgewehr 57 was adopted for Swiss military service in 1957 and replaced in 1990 by the lighter SIG SG 550 assault rifle, although some reservists used it for several more years. Design details The Stgw. 57 was developed during the late 1950s, internally named AM 55 at SIG. During its development, Rudolf Amsler was the principal designer at Schweizerische Industrie Gesellschaft. It is a selective fire rifle that employs a roller-delayed blowback operating system. The Stgw. 57 was used by the Swiss Army as a personal universal weapon. In Swiss service, it gradually replaced the K31 rifle, the Suomi M-31/Mp. 43/Mp. 44 submachine gun, the Lmg 25 light machine gun, and after 1974-1977 the Zf. Kar. 55 designated marksman rifle. Features The weapon is mainly made of pressed sheet-metal components to ease mass production and incorporates the "straight-line" recoil configuration. This layout, placed both the center of gravity and the position of the shoulder stock nearly in line with the longitudinal axis of the bore, a feature increasing controllability during burst or automatic fire. The Stgw. 57 has a distinctive T-shaped bolt handle similar to the earlier K31 rifle. As with all roller-delayed designs, the roller recesses and rollers in the receiver will eventually wear down, decreasing the bolt head to bolt carrier gap. Many roller-delayed blowback weapons accept this as the functional service life of the rifle, but the Stgw. 57 has interchangeable locking recesses, so they can be replaced when worn. The solid neoprene rubber, fixed butt-stock was designed with durability in mind and dynamically deforms to cope with the recoil produced by heavy rifle grenades. The Stgw. 57 sports a large carrying handle at its balance point that can be used during quick position changes or on the march. The ribbed handguard of the Stgw. 57 is also molded out of solid neoprene rubber. Barrel The Stgw. 57's barrel is relatively thick and rifled along of its length and has a