SIG SG 510

The SIG SG 510 or 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 SIG SG 510 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").
The Sturmgewehr 57/SIG SG 510 was adopted for Swiss military service in 1957 and replaced in 1990 by the lighter SIG SG 550, although some reservists used it for several more years.
Design details.
The SG 510 is derived from the AM55 developed during the late 1950s. 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 SG 510 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 SG 510 has interchangeable locking recesses, so they can be replaced when worn. 
The solid rubber, fixed butt-stock was designed with durability in mind and dynamically deforms to cope with the recoil produced by heavy rifle grenades. The SG 510/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 SG 510/Stgw 57 is also molded out of solid rubber.
Barrel.
The SG 510/Stgw 57 barrel is relatively thick and rifled along of its length and has a 270 mm (1 in 10.6 in) 4 groove rifling.
At the end of the barrel an integral long muzzle brake/flash hider is fitted that reduces recoil by about 25%. A rifle grenade-launching interface is also integrally machined into the body of the barrel. 
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.
The Stgw. 57 bipod legs are marked for use as an inclinometer for aiming rifle grenades at various ranges with and without an additional booster-charged rifle grenade. For such indirect rifle grenade fire, the Stgw. 57 was set on the ground and a weighted string attached on the sling loop was used as a plumb-line to set the correct firing angle reference mark on the appropriate bipod leg (left for boosted, right for unboosted). Swiss army knives were used as a makeshift plumb bob at the end of a string.
Trigger mechanism.
The trigger mechanism has a three-position fire selector switch that is also the manual safety toggle that secures the weapon from accidentally discharging. The user selects the operating mode with a large side lever on the left side of the trigger pack that can be rotated to select S (safe), E (semi-automatic fire) or M (full-automatic fire).
On the right hand side of the rifle, there is a foldaway 'winter' trigger which enables the operator to use the rifle with arctic mittens. It is also used for rifle grenade firing to avoid recoil-induced hand injuries.
Sights.
The SG 510/Stgw. 57 has a straight-line stock design, and an elevated iron sights line with a relatively long sight radius. Both the front and rear sights can be folded down when not in use.
The rear peep sight of the Stgw. 57 can be adjusted from . From the sight adjusts in increments. From in increments, and from in increments.
For low-visibility use, a reversible clip-on diopter with two self-luminous tritium-powered inserts (later carbon-14, one-side only) fitted laterally on each side of the aperture was issued. This element could be clipped by the user on the standard diopter. The front sight hood contained a permanently attached single tritium-powered (later carbon-14) insert above the post to complete the low visibility sight line. 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 employed circular error probable method cannot be converted and is not comparable to US military methods for determining rifle accuracy. When the R50 results are doubled the hit probability increases to 93.7%.
For anti-personnel use, the SG 510/Stgw. 57 typical maximum range for consistent accuracy is .
For designated marksman use, the SG 510 can be equipped with a quick-detachable Kern & Co Aarau 4×24 telescopic sight. The sight weighs and includes a variety of features, such as a Bullet Drop Compensation (BDC) elevation adjustment knob for ranges from 100 to 800 m, an tritium-illuminated reticle that enables target acquisition in low-light conditions and a diopter eyesight correction adjustment. Included with the sight is a lens hood with additional protective lens and a gray filter for glare reduction. A night vision sight with an infrared light can also be mounted. The Chilean version can fit a German-made Supra 4×24 telescopic sight.
Accessories.
The Swiss Army Stgw. 57 is fed from curved detachable box magazines, made from aluminum and containing 24 rounds weighing empty and fully loaded.
Special silver-colored magazines that can contain 6 grenade-launching cartridges (Gw. Treib.-Pat. 44) for use with rifle grenades weigh empty and fully loaded.
The PE 57 had 6-round magazines available as well.
Magazines with 20-round capacities for the 510-4 in 7.62 NATO and 30-round originally for the LMG 25 may operate in the Stgw. 57 as well.
Other accessories include a leather sling, a bayonet, various magazine pouches, cavalry holsters, and fortress-firing kits. 
Rocket-boosted rifle grenades can be launched without special provisions. Grenade-launching cartridges enable the SG 510/Stgw. 57 to fire "Gewehrgranaten 58" rifle grenades which weigh depending on the warhead type. These rifle grenades achieve a muzzle velocity of and a maximum range of without the help of a booster charge or a V7 of and a maximum range of with the help of a booster charge.
The rifle grenade 58 may be fitted with the following warheads:
Accuracy potential.
The following tables list accuracy statistics for typical in service Stgw 57 rifles firing 7.5×55mm Swiss GP 11 service ammunition obtained by average marksman. The statistics were computed under the 1950-ties Swiss method for determining accuracy, which is more complex than Western methods which usually involve firing a group of shots and then measuring the overall diameter of the group. The Swiss method differs in that after a group of shots is fired into the target from a machine rest hits on the outer part of the target are disregarded, while only half of the hits on the inner part of the circles are counted (50% or R50), which significantly reduces the overall diameter of the groups. The vertical and horizontal measurements of the reduced groups are then used to measure accuracy. This circular error probable method used by the Swiss and other European militaries cannot be converted and is not comparable to US military methods for determining rifle accuracy. When the R50 results are doubled the hit probability increases to 93.7%.
For reference a 1 minute of arc (MOA) circle at has a diameter of and at has a diameter of . The radius of a circle is half its diameter.
Civilian use.
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 semiautomatic only configuration. Dedicated factory-built, semiautomatic only rifles for private purchase were available and designated PE 57, PE means "Privat Einzelfeuer" ("private single fire"). These civilian PE 57 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 SG 510 rifles on the private weapons market is reported to vary between 300 and 500 Swiss francs.
In Switzerland the SG 510 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 the original military barrels (extremely long lasting: up to 25'000 rounds) could be replaced by so called match barrels similar to those used in standard single shot match rifles. Other upgrades include the tightening of the rifle in general, a lighter hammer, custom pistol grip and barrel jacket and regularly replacing the worn parts of the lock. A completely upgraded Stgw 57/03 almost equals a standard rifle in precision (results of 95% are common) and sells for 3500-4000 Swiss Francs.
SG 510-4 7.62×51mm NATO variant.
The SG 510-4 chambered in 7.62×51mm NATO with a 304.8 mm (1 in 12 in) twist rate barrel was adopted by Chile and Bolivia. Compared to the SG 510, the following changes were applied:
The SIG 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).