57 ) is a selective fire battle rifle designed by Schweizerische Industrie Gesellschaft (now SAN Swiss Arms) of Switzerland.

As an assault rifle, the model AM 55 entered service in the Swiss Army in three designations F. ass.

44 submachine gun, (iii) the Lmg 25 light machinegun, and (iv) in the 1974–1977 period, replaced the Zf.

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.

Many roller-delayed blowback weapons accept this as the functional service life of the rifle, but the Stgw.

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.

57 sports a large carrying handle at its balance point that can be used during quick position changes or on the march.

[1] At the end of the barrel an integral 26 mm (1.0 in) long muzzle brake/flash hider is fitted that reduces recoil by about 25%.

The barrel is surrounded by a perforated tubular aluminium barrel jacket with two spring detents for a sliding, underfolding integrated bipod—one near the muzzle to reduce shot dispersion during automatic fire, and another nearer the receiver and balance point offering more flexibility in the rifle role.

Swiss army knives were used as a makeshift plumb bob at the end of a string.

The trigger mechanism has a three-position fire selector switch that is also the manual safety toggle that secures the weapon from accidentally discharging.

[2] When not in use, the low-light diopter is stored inside the pistol grip cavity, with two blister packets of weapon grease.

The useful life expectancy of tritium-illuminated night sight inserts is roughly 7-10 years.

According to the Swiss Army, the 50% windage and elevation dispersion shot at 300 m from a machine rest averages 6 cm (2.4 in).

The sight weighs 730 g (26 oz) and includes a variety of features, such as a Bullet Drop Compensation (BDC) elevation adjustment knob for ranges from 100 to 800 m, a tritium-illuminated reticle that enables target acquisition in low-light conditions and a diopter eyesight correction adjustment.

Other accessories include a leather sling, a bayonet, various magazine pouches, cavalry holsters, and fortress-firing kits.

57 to fire Gewehrgranaten 58 rifle grenades which weigh 1.16 to 1.18 kg (2.56 to 2.60 lb) depending on the warhead type.

Rifle grenade variants include: Upon completion of their military service, members of the Swiss armed forces could obtain ownership of their personal Stgw.

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.

When the original sighting is kept Swiss sport shooters refer to a such unmodified rifle as Stgw.