The Sturmgewehr 57 (Stgw. 57 ) is a selective fire battle rifle designed by Schweizerische Industrie Gesellschaft (now SAN Swiss Arms) of Switzerland. The Stgw. 57 assault rifle uses a roller-delayed blowback system similar to the blowback system of the Heckler & Koch G3 (H&K G3) and CETME rifles. As an assault rifle, the model AM 55 entered service in the Swiss Army in three designations F. ass. 57 7.5mm (Fr: Fusil d’Assaut; Ital: Fucile d’Assalto 1957) and 7.5mm Stgw. 57 (Ger: Sturmgewehr 1957). Technologically, the Stgw. 57 was the mechanical and design basis for the export-variations of the SG 510 family of small arms. After thirty-three years, from 1957 to 1990, the Swiss Army replaced the Stgw. 57 with the SIG SG 550, a lighter-weight assault rifle.

The Stgw. 57 assault rifle was developed during the late 1950s, with the in-house model name of AM 55, at SIG Combibloc Group (SIG). During development, Rudolf Amsler was the principal designer at Schweizerische Industrie Gesellschaft. Mechanically, the Stgw. 57 is a selective fire rifle that employs a roller-delayed blowback operating system. In military service, the Swiss Army issued the Stgw. 57 as the personal weapon of every soldier. In the course of service, the Stgw. 57 replaced the following four weapons: (i) the K31 rifle, (ii) the Suomi M-31/Mp. 43/Mp. 44 submachine gun, (iii) the Lmg 25 light machinegun, and (iv) in the 1974–1977 period, replaced the Zf. Kar. 55 sniper's rifle.

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.

Upon completion of their military service, members of the Swiss armed forces could obtain ownership of their personal Stgw. 57 rifle by paying an administrative fee. These "civilianised" Stgw. 57 rifles were converted to a semi-automatic only configuration. Dedicated factory-built, semi-automatic only rifles for private purchase were available and designated PE-Stgw. 57, PE means Privat Einzelfeuer ("private single fire"). These civilian rifles incorporate subtle design changes to avoid some cross-compatibility with army rifle parts. As of 2007, around 40 percent of discharged soldiers choose to retain their weapon (nowadays the SIG SG 550), and the going rate for civilianised Stgw. 57 rifles on the private weapons market is reported to vary between 300 and 500 Swiss francs.^[4]

In Switzerland, the Stgw. 57 is also used for target shooting matches. For this the standard iron sights can be replaced by target shooting diopter and globe sight sighting lines. When the original sighting is kept Swiss sport shooters refer to a such unmodified rifle as Stgw. 57/02 (small diopter allowed, but no globe sight). When a globe sight is added and possibly the sighting line radius is lengthened by mounting the globe sight nearer to the muzzle it is referred to as Stgw. 57/03. As of 2017, original military barrels, which are known for longevity and are rated for up to 15,000 rounds (for later nitrided variants), could be replaced by match-grade barrels. Other upgrades include improved target sights, mirage bands, lighter hammers, custom pistol grips, custom barrel jackets and regular replacement of bolt components.

The SG 510-4 chambered in 7.62×51mm NATO was adopted by Chile and Bolivia. Compared to the SG 510-1, the following main changes were applied:

• Lighter, shorter (505mm) barrel with 8 (early), or 16 (late) chamber flutes, 304.8mm (1 in 12 inch) twist rate
• Revised delay angle on the bolt carrier wedge
• Straight 20-round box magazine with feed path insert to accommodate for the shorter cartridge
• New low-profile iron sights, with ramp-adjustable rear sight aperture and semi-hooded front sight
• Spring-loaded detent on the front barrel jacket bushing/front sight base for rifle grenade sight
• Angled stock tube and stock assembly, with correspondingly modified recoil spring assembly and bolt carrier tail
• Walnut wood stock and handguard
• New pistol grip design (after ca. 1966)
• Lightened roller recesses and winter trigger
• Simplified bolt carrier with add-on, unified rubber buffer and charging handle lug

The SG 510-4 is officially classed as an automatic rifle but also served as a designated marksman weapon (in Chile with Supra 4×24 scope). ^{[citation needed]}

• AM 55: Original version as adopted by the Swiss Army as "7,5mm Stgw. 57".
• SG 510-1: Export model of the AM 55 chambered in 7.62x51mm NATO. ^[5]
• SG 510-2: Lightened variant of the SG 510-1.^[6]
• SG 510-3: 7.62×39mm variant with shorter barrel, receiver and barrel jacket. This was produced in small numbers as a prototype and submitted to the Finnish Army. This model of the rifle was never mass-produced.^[6]
• SG 510-4: 7.62×51mm NATO variant used by Bolivia and Chile. This version uses a shorter barrel and a buttstock made from wood rather than rubber. Overall length is much less than the Stgw 57 with a subsequently lower weight. The buttstock and stock tube are angled downward rather than straight inline with the receiver.
• SG 510-5: .30-06 Springfield (7.62×63mm) experimental rifle made for testing by the Mexican Government.
• SG 510-6: 7.5×55mm Swiss test batch of rifles based on the SG 510-4 in the Swiss military chambering intended to replace the heavier Stgw 57. The rifles were only built for testing and were no longer considered after 1980s trials.
• SG 510-7 T / SIG-AMT: semi-automatic only variant of the SG 510-4 imported into the United States in relatively small numbers. It was available in .308 (7.62×51). "AMT", term used after 1969, stood for "American Match Target". It was equipped with fine wooden furniture and could be ordered with a rounded upper handguard.
• Gewehr 2 (G2): 7.62×51mm NATO variant procured by the Bundeswehr for trials in 1956, which eventually lost to what would become Heckler & Koch G3.
• PE-Stgw. 57: (Privat Einzelfeuer) semi-automatic only civilian version of the AM 55 available in 7.5×55mm GP 11 Swiss. This variant is not the same as privatized former Swiss Army service rifles. It was specifically built as a semi auto only rifle along the lines of the Stgw. 57 with which it is frequently confused. While over 700,000 of the Stgw. 57s were built for military use, less than 5000 PE-Stgw. 57s were made.

•  Bolivia: SG 510-4 variant.^[7]
•  Monaco: Used by the Compagnie des Carabiniers du Prince.^[8]
•   Switzerland: Adopted by the Swiss Army in 1957.^[6]
•  Ukraine^[7][9]
•  Chile^[7]
•  Zaire^[10]

• Stgw 57 with iron sights, bayonet and folded bipod. On display at Morges castle museum.
• Stgw 57(/03) modified for sport shooting with a diopter and globe sight sighting line.
• Receiver of a Stgw 57, seen from the right.
• Receiver of a Stgw 57(/03) modified for sport shooting, seen from the left.
• Stgw 57 dismantled.
• Stgw 57, bolt, recoil spring.
• Stgw 57 trigger housing with winter trigger extended.
• Stgw 57 bayonet and frog.
• Stgw 57 cleaning kit pouch.
• Left to right; boosted and unboosted orange training and a Gw HPz G 58 armour piercing rifle grenades
• Gw HPz G 58 armour piercing hollow charge rifle grenade poster
• Gw St G 58 fragmentation rifle grenade poster
• Gw Nb G 58 smoke rifle grenade poster

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