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Caliber: 7.5x55 mm GP11 (Stgw.57) or 7.62x51mm NATO (SIG 510-4)
Action: roller retarded blowback
Overall length: 1105 mm (1016 mm SIG 510-4)
Barrel length: 583 mm (505 mm SIG 510-4)
Weigth: 5.56 kg empty (4.25 rg empty SIG 510-4)
Magazine capacity: 24 rds (20 rds SIG 510-4)
Switzerland began to experiment with intermediate cartridges before the World War 2 and, being a neutral country, closely watched the developments made during and after the war. Being entirely satisfied with the power and accuracy of its 7.5 mm GP11 (7.5x55) cartridge, the Swiss army tried to achieve a full power selective-fire rifle. After a couple of false starts, first with the gas-operated Sk-46 self-loading rifle, and secondly with the most unusual AK-53 blow-forward design, the famous SIG company finally produced a weapon which satisfied the Army in 1955.
This was the 7.5 mm AM-55, a retarded blowback design, developed under the leadership of the Rudolf Amsler. The basic principles of the action were borrowed from German WW2 period Mauser 'Gerät 06H' and Stg.45(M) assault rifles, but with much altering involved. In 1957 the Swiss army adopted the AM-55 as the Schturmgewehr-57, or Stgw.57 for short. Made between 1957 and 1983, the Stgw.57 represented one of the finest and most expensive automatic rifles ever issued to any army in the world. Chambered for full power 7.5 x 55 GP11 ammunition, the Stgw.57 provides long range accurate shooting in semi-automatic mode, necessary for the typical Swiss mountain country, in combination with significant full auto firepower, thanks to its relatively heavy weight, integral bipod and a shrouded barrel. In the modified form, known as the SIG-510, this design was relatively successful, being sold to various South American countries, most notably Bolivia and Chile, chambered for 7.62 mm NATO ammunition.
The action of the Stgw.57 was derived from the roller-retarded blowback system originated by Mauser engineers in Hitler’s’ Germany. However, the Swiss designers replaced the rollers with the roller-shaped pivoting flaps, interposed between the bolt head and the bolt body. The receiver is made from stamped steel, with a separate trigger unit housing made integral with the pistol grip frame and trigger guard. The fixed barrel has a perforated steel jacket with two mounting points for an integral bipod – one near the muzzle, and another near the receiver. The front part of the barrel is exposed to act as a rifle grenade launcher. To smooth out the excessive recoil generated in full automatic fire, and especially by rifle grenades, the fixed butt-stock is fitted with a recoil buffer. The safety / fire mode selector is located at the left side of the trigger unit. Stgw.57 is fitted with large, T-shaped charging handle and with the folding “winter trigger”, which, when unfolded, extends down below the trigger guard, enabling the rifle to be used in arctic mittens. Since the Stgw.57 was designed with the so-called straight-line layout, the raised sights are mounted on high, folding bases, with the rear sight being micrometer-adjustable from 100 to 650 metres. The Stgw.57 also could be fitted with the special Kern 4X telescope sight. Stgw.57 is fed from curved box magazines, made from steel and containing 24 rounds. The small forend is made from plastic and the gun is fitted with a side-folding carrying handle. Other accessories include the sling, the bayonet, and a special small-capacity magazine for blank grenade-launching cartridges.
Export military versions of the Stgw.57, known as SIG SG-510, were made in 4 basic modifications, of which only one was made in any significant quantities, the SG-510-4. This was chambered in 7.62 x 51 NATO, had a shorter barrel, and non-folding aperture sights. The forend and buttstock were made from wood.
Other versions included the SG-510-1 (exactly the same rifle as the Stgw.57), the SG-510-2 (a lightweight modification of the Stgw.57, also in 7.5 mm), and the more compact SG-510-3, chambered for the Soviet 7.62 x 39 cartridge (produced in late 1950s for assault rifle trials in Finland). Civilian semi-automatic only versions of the Stgw.57 were designated as PE-57 (in 7.5 mm GP11) and SIG AMT (a semi-auto version of SG-510-4 in 7.62 mm NATO).
