The Stgw 57/Fass 57/SIG 510 Factory Cutaway:
Hi all! I stopped by the Lausanne gun show two weeks ago and picked up this beauty:
It is a factory original Swiss Army Stgw 57 demonstration cutaway! The gun is in decent condition, with some patina and red paint missing, but is mechanically excellent. Let's dive deeper into the wonderful design of this rifle.
An original Swiss Army Sturmgewehr 57. Note the rubber buttstock, folding sights, bipod and leather sling.
The Stgw 57 is an interesting battle rifle. The rationale behind this gun was to arm the infantry with an "universal weapon" that would replace the bolt-action rifle, machine gun and machine pistol. It was selected in late December 1956 over the Waffenfabrik Bern competitor for cost reasons (the SIG prototype costed only 495 CHF to produce over the 1100 CHF that the W+F Bern model required). It incorporates a modified roller-delayed blowback mechanism inspired by the StG45/Gerät 06 H prototypes, folding sights from the FG42, and a buttstock socket just like the famous MG42. In order to reduce production costs, SIG used innovative production techniques and rubber/polymer materials for the rifle's construction, in an effort to minimise the number of machined parts. Of course, being a Swiss rifle, no compromises were made on the quality of the construction and overall robustness. Because of its heavy, welded, machined, stamped and brazed construction, the Stgw 57 weighs a whopping 6,5 kg fully loaded! Not only is the rifle one of the heaviest service weapon of the world, it is also one of the most expensive! Each Stgw 57 costs the Swiss government a grand total of 1000 CHF (of which 495 CHF are production costs and 75 CHF accessories).
The cutaway here is done off of a mixture of mismatched parts. SIG or some other entity probably gathered up what's available, cut them up and assembled them as training aids. Apparently, they were training aids for SIG armorers and enlisted recruits. There is also a variant of this cutaway with a full size barrel and corresponding fittings. The cutaway job itself is extremely well done. Most components (including takedown pin and magazine) have windows milled into them to show their function and are highlighted in red lacquer/paint.
The most dramatic change is the shortening of the barrel. It was chopped off to a mere 10cm stub, faced off, chamfered and a quarter of it milled away. because of that, there is no handguard, shroud, bipod or front sight. The receiver has most of the right side milled away. The bolt head and body also have large cuts displaying the firing pin channel and left roller. Note the combination ejector/extractor, fluted chamber, "beer keg" charging handle, loaded chamber indicator, charging handle lug, replaceable mortices retained by c-clips and the barrel that needs a tightening . The front and rear trunnions are spot welded then hard soldered/brazed to form an insanely strong and seamless joint. Notice the "-" marked rollers. This signifies a dimension of 11.96mm. Sets of rollers are available from 11.92mm to 12.08mm, in .08mm increments to adjust headspace. The loaded chamber indicator screw and spring are clearly visible here.
Top view of the cutaway rifle. You can clearly see the unusual firing pin and hammer arrangement. Because of the recoil spring and bolt layout, the hammer is not capable of striking the firing pin directly. Instead, the hammer is offset to the left and a L-shaped transfer lever arranged. The clever SIG engineers found out that a firing pin retainer is now no longer necessary, as the lever does the job. The AR15 design has the same issue, but has a long, hollowed out bolt carrier allowing the hammer to pass through and strike the pin, not unlike the StG44. Also note the famously Swiss "beer keg" charging handle, a tradition dating back from 1889. You can also see the remnants of the Swiss crest on the front trunnion, the unfolded diopter rear sight, the profile of the roller recess and the window cut out on the buffer tube.
Alternate view with an inert 7.5x55mm GP11 cartridge chambered. Note the indicator now sticking up from the front trunnion. An interesting feature unique to the Stgw 57 is the step right before the first shoulder of the chamber. This allows the case to deform slightly(visible here), cushioning the blow of the bolt to avoid bounce. This is in my opinion a more elegant solution to the awkward G3 style locking lever.
A great view of the Stgw 57's ejector in action. This is one of the most unusual methods of ejection I have ever encountered. The heart of this system is the combination extractor claw/ejector, which is the dark triangular piece powered by a funky looking spring. The case is initially held by a cartridge tensioner (often mistaken as a second extractor) and the extractor. When the extractor strikes a ramp integral to receiver (seen here), it pivots and swings the case 90 degrees out of the ejection port, rim first. The benefits of this system are multiple. The ejection force is very mild and consistent for such an inherently violent action, resulting in a neat pile of brass less than a meter away from the shooter. This is critical for the mandatory shooting sessions in tight and clean shooting ranges in Switzerland. Violent ejection is quite a nuisance in these premises. This completely opposes the G3, which uses the same operating system, but has a fixed ejector that flings out casings into orbit.
The trigger mechanism is very similar to the one found in the M1 Carbine and G3/MP5 family, except that there is a dedicated disconnector instead of a shoulder on the trigger. The basic principle however, remains the same. The sear has an oblong hole that allows forward and backwards movement. The hammer forces the sear back when cocked, aligning the rear of the sear with the trigger (in the case of the Stgw 57, the disconnector). The safety blocks upwards movement of the trigger, and a winter trigger is present, engaging a pin on the side of the trigger blade. The automatic function allows movement of the auto-sear, which presses down on the sear every time the bolt closes.
Original line drawings of the trigger mechanism in cocked-uncocked state. Note the position of the sear in relation to the disconnector.
Right view of the trigger pack of the cutaway Stgw 57, winter trigger engaged, hammer cocked. The cutaway windows on the frame and trigger itself clearly show the arrangement of fire control components. Note the absence of the auto-sear. This rifle's fully automatic has been deactivated by the certified armorer. The top tail of the sear has been cut off, the auto-sear hand removed, and the operating shoulder on the bolt body milled away. Such practice is common in Switzerland signifying decommissioning of the rifle. Note the triangular section of the sear. It engages against a set screw used to adjust let-off and weight. The screw had to be adjusted in such a way that when the second stage is felt, i.e when the triangular section strikes the set screw, the sear and hammer must be barely engaging. This results in a crisp and responsive trigger pull. Unlike the later Stgw 90/SIG 550, there is no provision for easy adjustment of overtravel. The white plate locking the rifle in semi-auto also seems to be missing here.
Left detail view of the trigger pack. The fire selector has three positions: S (Sichert, Sicherheit), E (Einzelschuss/Einzelfeuer) and M (Mehrschuss? Mitrailleuse? Mitrailler?). You can just barely make out the disconnector and its spring through the trigger window. Notice the oblong hammer pin hole. This is damage due to dry firing.
Top view of the trigger pack. Pretty straightforward and spacious layout. From left to right: fire selector, hammer, hammer strut and pin, sear (partially cut off), trigger, winter triggerand pin retaining plate.
Trigger pack, completely disassembled except for the magazine catch and hammer assembly (roll pins and riveted pins, no chance). The hollow pistol grip is made out of black glossy plastic and is fixed to the trigger pack frame though two screws and polyamide-lined safety nuts. The internal storage compartment may house personal effects, a night sight or blister packs of Automatenfett, a standard issue black molybdenum/graphite grease. The compartment is sealed off by a sliding trapdoor actuated by a bullet tip. The trigger pack frame is constructed out of pressed and welded sheet steel. Notice the cutaway window on the grip and the pin retaining plate on top of the fire selector lever. The whole trigger pack sits on two stamped out projections or shelves on the receiver and is retained by a very interesting push-pin, which is also cut away in this model.
This type of push pin is self-locking, meaning that it is firmly locked in place and requires actuation of a button to remove. The principle is clearly demonstrated by this cutaway. The two extensions of the spring keep the pin in the gun and are drawn inwards when the button is pressed. This pin is quite complex for its task, but was nonetheless carried over to the Stgw 90/Fass 90 rifle.
Let's now move on to the heart of the system: the bolt. As said earlier on, the Stgw 57 utilizes a roller delayed blowback system. This mechanism works through mechanical disadvantage of the bolt head through sloped surfaces and rollers, allowing safe extraction and a relatively light bolt group.
The rectangular bolt group is composed of two main parts: the head which houses the rollers, extractor/ejector and cartridge tensioner, and the body, which comprises of the critical cam surfaces, firing pin and transfer lever. The bolt is a hefty piece of precision cast and machined steel which has a mass of 600g and a total length of 126.5mm. The picture above is the cutaway bolt in the unlocked position. The rollers are retracted flush into the head.
The underside of the bolt group. The shoulder responsible for tripping the auto-sear hand has been ground off as part of the decommissioning process. The cartridge tensioner is clearly visible at the bottom. Although it looks and functions like an extractor, there is no actual hook. Instead, the tensioner acts through spring tension and static friction to hold the case firmly against the bolt face.
The bolt group 100% disassembled. The rollers are actually guided by two secondary bearing pieces that are maintained by a ball bearings in the bolt head. The cartridge tensioner left of the extractor has the role of pressing the cartridge case against the top of the bolt face recess, creating friction. The extractor/ejector itself has an internal spring steel bracket that provides support for the spring. The firing pin is fluted to avoid the piston effect during a popped primer. Notice the precision roller cam surface on the bolt body. They are inserts made out of harder, more expensive steel that are welded to the bolt body itself. This drives the cost of the part down, as it is not required to make the whole thing out of expensive, higher grade steel. This approach was also adopted on the SKS rifle.
SIG engineers went through great lengths to reduce felt recoil on the Stgw 57. This was done through an in-line layout, muzzle brake, rubber buffer, soft rubber buttstock and complex recoil spring assembly. All these elements contribute to the 40% in recoil reduction compared to the previous generation K31.
Here is a closeup of the socket attachment of the buttstock inspired from the MG42. Two witness marks are carved on the 12 and 2 o'clock positions of the buttstock tube to aid reassembly. An interesting feature of the Stgw 57 is the accordion-shaped rubber buffer located at the very rear of the buffer tube (it can be seen at the bottom of the cutaway window). The role of the buffer is to cushion the blow of the bolt group during the return stroke. A projection machined onto the bolt body (pictured here) engages the buffer.
The buttstock assembly is an interesting piece of hardware. It is made out of solid rubber material. Notice the "59" mould stamp, signifying 1959 manufacture. On the buttstock or buffer tube is the spring catch responsible for the retention of the whole thing. The troop tag, composed of a plasticised cardboard and clear plastic sheet, is enclosed in a pocket on the tube. The tag is only accessible by removing a set screw and unthreading the buttstock.
The entirely captive telescopic recoil spring assembly is a complex arrangement of three spring and several guide rods and tubes. The primary recoil spring is the thinner one whereas the squiggly large is a buffer spring, decelerating the bolt group gently to reduce felt recoil. A third spring is hidden inside of the assembly and powers the very tip of the spike-shaped plunger (see line drawing). It is not entirely clear why SIG engineers did this, but my theory is that this was done to aleviate excessive spring oscillation during the violent actions of the bolt group.
Naturally, since this is a Swiss rifle, the sighting system is very well made and arranged. With its whopping 700mm or so sight radius, precision adjustable rear aperture and globe front sight, the Stgw 57 will easily shoot beyond its effective range of 500 meters. The diopter is especially interesting in the Stgw 57, and is naturally, also cutaway!
The diopter is elevation and windage adjustable. These adjustments are quite soldier friendly, as the windage screw fits a Swiss Army knife screwdriver blade perfectly! The diopter tower itself is very complex, topping off at a whopping 22 parts! It snaps positively in either the folded or the combat ready position. The elevation is adjustable from 100 to 640 meters in progressive 50, 30 and 20m increments, which is very precise. A knurled drum retained by a spring loaded latch is rotated to adjust elevation. Windage is adjusted through a worm gear (shown here) that meshes with the aperture hole itself. Notice that on the cutaway, the sight is set on the "red 3", which is the 6 o'clock hold setting equivalent to the 380m 12 o'clock hold. This setting is used as a battle sight and as a target shooting setting.
The 24-round magazine for the Stgw 57 is especially well made. The panels are milled (!!!) to exacting dimensions and the lugs precisely machined for fit. While it may seem smooth on the outside, the magazine shell has two internal rib/swelling that reduce the friction required to push the rounds up and strengthen the magazine. Several patterns of magazines exist, but the majority have a precision moulded polymer follower and stapled lugs, whereas earlier variants had steel followers and spot-welded lugs. The cutaway Stgw 57 comes with a special cutaway magazine with two milled and painted windows to show off the stacking and spring.
Cutaway mag next to an ordinary mint condition magazine. Both have the same polymer follower and stapled lugs. Notice the milling marks and the characteristic "hump" on the last third of the magazine body.
The feed lips have excellent geometry and are very strong. The polymer followers are precision moulded and are very well finished. Notice the two reinforcement ribs right before the shoulder of the profile.
Cutaway magazine is on the left. Comparison of manufacture stamps. I do not know who they stand for (if somebody can elucidate on that Notice the difference in finish. The left magazine has been sanded or ground whereas the right one is left raw.
As an extra, here is a picture of a full clip of six GP11 "Manipulierpatrone" or dummy cartridges, and an inert GP11. Solid brass construction, very well made. However, my only gripe are the knurled recesses on the dummies. The loaded chamber indicator tends to grab them.
Here is the cutaway model field-stripped. The Stgw 57 is very easy to care for and disassemble, thanks to its large, captive assemblies.
And to end with a high note, here is the Stgw 57 fully disassembled. Or mostly, some assemblies have roll pins and bumped, staked and riveted pins. A punch and a screwdriver blade were used to complete this:
I hope you enjoyed this presentation of this wonderful rifle and some of its features. I wish you a happy New Year and a wonderful holiday season!