SIG AK-53: A Truly Weird Forward-Operating Rifle

One of the most mechanically unusual rifles I have had the chance to examine is the SIG AK-53. One of a long line of interested experimental self-loading designs made by SIG between the 1920s and 1950s, the AK53 is a gas operated rifle with a fixed breechblock and a barrel that cycles forward when fired. The gas system compresses a spring rearward first, which then releases and pushes the barrel forward in a system with a few elements in common with the British Farquhar-Hill rifles and machine guns. The magazine is also unique in design to allow rounds to move directly upward so as to have the barrel drop over them in a reverse sort of feeding (the magazine bears some vague similarity to the Madsen and Mendoza LMG magazines in this way…a bit). For reasons which will become very clear upon disassembly (if not already made clear by that description), only prototypes of the rifle were ever made, with no series production.

Many thanks to the Royal Armouries for allowing me to film and disassemble this very rare rifle! The NFC collection there – perhaps the best military small arms collection in Western Europe – is available by appointment to researchers. Anyone, however, can browse the various Armouries collections online here.

70 Comments

  1. This rifle redefines reverse engineering. Its almost like they took it up on a dare to see if they could create an operating system that works counter to all logic. Kudos to them, but seriously, what could go wrong?

  2. I’m wondering exactly how the barrel locking mechanism functions in this rifle. You showed the mechanism (flapper locked) early in the video but subsequently made no mention of how it was engaged/disengaged. From the description of the how the “blow forward” aspect of the rifle operated it almost seems like the locking mechanism is superfluous except for the fact that the rifle is chambered in a caliber that would seem to require a locking mechanism. But then I go back to the fact that the rifle has two HUGE operating springs and wonder if perhaps it could operate as a blowback (blowforward?) system.

    • The locking flaps aren’t attached to the barrel, but the sliding piece around the barrel. The barrel should begin to move forward upon firing so it might be a delayed blow forward, IMHO.

          • Gas operated, flapper locking, moving barrel. I think it could designated so. No blowing forward, because the movement of the barrel rely on springs (compressed by gas piston) to go forward and back. It shares with blowforward action the feeding system with stationary breech and moving barrel.

          • Dansquad: As best I can tell when the gun fires the sliding sleeve around the barrel is locked into the receiver. When the sleeve is fully forward relative to the barrel the flaps are forced outward but when the sleeve is rearward on the barrel the flaps can retract but aren’t required to do so. You can see the the lower flap just hanging in the video. As such, the only thing holding the barrel against the breech is the inner barrel spring. The sleeve can’t move forward, but the barrel can.

          • The author should stick in adverts, after 189 times; i’ll want a pair of cloogies “new imaginary footwear” and I will never know why.

          • Like watch you can power off “tugging” or such… I mean, it’s an invention that might possibly be needed.

          • A fine Swiss watch, fed by Razzle… Kind of utilitarian I suppose; but not in the German, makes sense if you think French also.

          • By sleeve you mean the gas piston. Barrel retained back by its own spring and locked to receiver by flaps. Sleeve/gas piston with its spring does not interact yet upon firing. Gas port vents gas pressure, sleeve/piston retracts, comprising its spring and barrel spring until engaging barrel spring stop by its front prongs. When gas pressure lowers, sleeve/piston goes forward by its spring tension, pulling the barrel with comprised barrel spring as a single unit, flaps disengage. Extraction, ejection. Sleeve/piston pulling the barrel reaches receiver front stop part, disengaging sleeve/piston from barrel spring-stop prongs. Sleeve/piston at rest, and barrel goes back under its own spring tension, feeding and locking again by flaps. Overcomplicated system. Truly swiss

          • I don’t mean piston, there is a piece separate from the barrel to which the flaps are attached to. What unlocks them? I think they get bulled out of engagement by the barrel blowing forward.

      • The locking flaps ARE attached to the barrel. It seems you mistook the back of the barrel/chamber area with the sliding piece, that is in front of it and comprises up to the collet (barrel running inside it). It is just the opposite way. You can see it clearly at 16:57, looking at the collet. The barrel moves a bit out the collet when the flaps go out, as Ian pushes the back of the barrel against the sliding part with the enclosed barrel spring and collet. Hope you can see it clear now

  3. “For reasons which will become very clear upon disassembly (if not already made clear by that description), only prototypes of the rifle were ever made, with no series production.”
    I am wondering who designed it that way? Or maybe rather why decided to use such system? I suspect that such heavy mass (barrel) moving back-and-forth would result in humongous spread when fired without rest, especially in full-auto mode, if selective variant would be crafted, so it suggest that designer was absolutely no caring about minimizing spread of potential full-auto variant. Yet 30-round magazine is hinting that he was considering selective-fire version.
    Mind boggling-question is cyclic Rate-of-Fire of that weapons? I presume that “spring accumulator” was lowering Rate-of-Fire somewhat, but on the other hand Beardmore-Farquhar https://modernfirearms.net/en/machineguns/great-britain-machineguns/beardmore-farquhar-eng/ wasn’t so slow at 450 rpm (for comparison Lewis 550 rpm).
    Anyway, one we know: designer of that weapon, is mostly probably, type which do not choose simple ways.

    • I think it would be good in a system in which the reciever aims to be as short as possible, maybe something like a squad support weapon, or, in a higher caliber a heavy MG, bullpup’d to capitalize on maximum barrel length, while keeping the reciever very small. Throw a water jacket on there for maximum tacticool and volume of heavy sustained fire.

  4. Rifle with barrel moving forward-backward? Wait, I know
    Автоматическая винтовка системы Мощевитина образца 1934 г.
    Crafted in 1930s, barrel was locking into receiver by slight rotation, when cartridge was fired, bullet going down the barrel, not only “push” it forward but also apply torque, in this weapon it was used to unlock. Thanks to buffer into stock, its recoil was deemed “unpleasant but acceptable”, magazine was for 5 rounds, rifle mass 4 kg.
    Tests revealed that Rate-of-Fire of this weapon is 1400 rpm (very high for such kind of weapon), designer was ordered to work at “special purpose machine gun, which was deemed more actual need” so this rifle was left unfinished.
    description in Russian: http://commi.narod.ru/txt/markev/532.htm

  5. The example in the video sure is funky.
    The selector switch on the side is to choose between semi (E) and auto (M).
    Safety works just like a K31: with the ring turned and brought in the rearward position it prevents the firing ping from protruding enough to hit primers.

  6. Was “collet” the word that you were looking for?

    I’ve been interested in the AK53 for a dozen years or more. I didn’t realise that there’s one within two hours drive of me!

    And what an interesting little Farquhar it is 😉

    It doesn’t seem like it has taken much inspiration from the either the feed or the cocking systems used by the Schwarzelose blow forward pistol.

    It also fails to make full use of the savings available in forward action in both length and in total weight.

    Using forward action, you shouldn’t need any more standing breech than an Anson and Deely (boxlock) shotgun or rifle. Theven AK53 standing breech assembly is way too long.

    It is necessary to have weight in the reciprocating parts of a self loading action, to provide inertia and momentum to carry the parts through their cycle.

    A note worthy example of this is the other, more famous AK system: Kalashnikov’s.

    MTK improved on the Garand by straightening out the gas system and operating rod, making them more rigid, and putting plenty of weight into the bolt carrier.

    A rifle barrel is already heavy, it has to be to resist the internal pressure and to provide the required rigidity.

    Why not use that weight that you already have to have in the barrel? Why not get it to do all of the moving?

    Feed systems have moved on since the turn of the twentieth century Schwarzelose pistol and the mid twentieth century AK53

    The end twentieth century Nikonov AN94 with its fancy pulley system, potentially makes a way for a forward action to use conventional mags (heaven forbid, even that shitey, hind leg of a dog, united state thing!), and with minimal weight.

    An amazingly interesting gun and a system that still has interesting potential

  7. It really is a suprise that SIG manage to design this rifle in such an efficient manner.In before I always thought they must done some insane engineering to achieve a lock breech forward operating action, I’m wrong.AK-53 is really not that complicate.From field strip standpoint, the procedure of disassembly is acceptable, though obviously not refined enough.
    It’s interesting that if Swiss decide to operate the barrel with a muzzledevice instead of a Farquhar-Hill like action, the action can be even simpler.
    Maybe they figured that will make recoil even worse?Because it essentially is launching the barrel forward.
    I would guess that the recoil of AK-53 might felt really like a Chauchat,except the barrel assembly ram the receiver forward, thus help pull down the muzzle instead of pushing shooter backward.
    Any thoughts on that?

    • I haven’t thought it through yet,

      In practice it might take several years, or even several decades. The gears in my head turn slowly (and yeah, I’d love to make a Bilgram gear shaper.

      As an early guess, the gas system accelerating that big heavy gas piston back to compress the barrel opening spring, might do a lot to alleviate felt recoil

      The AK53, might already be quite soft shooting!

      With a bit of development in that specific direction (comfortable, controlable aimed shooting), I’m guessing that a forward acting locked breech rifle could be very good indeed.

      The counterweight Kalashninovs are supposed to be excellent. Perhaps the “other AK” could be even better?

      • According to the trial report on this site,AK-52 is already quite controllable under full auto, I don’t think AK-53 will do worse.

        Let me do some basic analyze on recoil:
        1.A large portion of recoil comes from the counteraction, which is compensate by muzzle brake and the power needed to operate gas piston.
        2.The backward momentum of gas piston convert into spring potential,then barrel spring push the barrel forward.The potential barrel spring has is only a portion of what gas piston have collected, while barrel have a larger mass than gas piston, thus create a mild backward push.
        3.Barrel slams forward, result in a forward push.
        4.Recoil spring push the barrel backward.Because the potential recoil spring has is only a portion of what gas piston have collected, while barrel have a larger mass than gas piston, thus create a mild backward push.

        I can imagine how controllable it is.

  8. Man, first weapon that’s compelled me to comment. Clearly there was some specification they were trying to meet, but for the life of me I can’t figure out what. Perhaps to cram as many moving parts into an assault rifle as humanly possible? I’d bet 50 bucks there was some pin or operating rod that got lost a long time ago and that’s why it doesn’t cock.

    Clearly this some someone’s baby, someone of some hefty means or influence, and no one took them aside and said, “This isn’t working”

    The whole thing is so weird that even Ian is struggling to think of similar designs.

    • As Ian has said,I don’t think AK-53 is that complicate.
      Only Barrel and gas piston is moving, the bolt and firing assembly is fixed in the receiver.
      I can definitely see this gun working.

  9. E must mean Einzelfeuer, ie single shot, but M, maybe Machinenfeuer, ie machinefire ??
    Weird but cool thing, a machinists wet dream 🙂

  10. Which long arm seems most impractical for mobile assault?

    1. SIG AK-53
    2. Colt R75A given a custom-built 50 round drum
    3. Water-cooled Fedorov Avtomat
    4. Chauchat-style select-fire .410 shotgun with 20-round detachable magazines full of plated buckshot
    5. Belt-fed AR-10
    6. Thompson M1921 with 100 round drum and pistol fore grip
    7. Any double-barrel bolt-action nitro-express rifle

    Don’t feel pressured to answer, but thanks for reading…

    • Hmmm…
      While not ideally suited to the modern battlefield, I’d gladly take the R75A or M1921. If by a “belt-fed AR10”, you mean something like the FightLite conversion for an AR15, but in 7.62, I’d use that, too.
      A regular Federov, sure, but add the water jacket & all associated paraphernalia, meh, not so much.
      Given the fussy nature of the Chauchat, no. Assuming you made a modern reproduction that worked properly in .410, still no. The little .410 holds 3-4 000 buck, so it’s effective capacity is a 7 round 12 gauge. (Now a Saiga with a 20 round drum would be most effective for room clearing & other such close in work.)
      A double barrel bolt rifle in .600 nitro? Absolutely! All armies should be equipped with them! In reality, I think I’d rather take one of those into combat than the AK53. This contraption looks like it’d be awkward to use, unreliable and difficult to maintain. I can’t imagine any scenario where the AK53 would be even a remotely practical option.
      You forgot to add the Cobray Terminator to your list… 😉

      • I suppose I should have added a Nambu Type 100 SMG to the mix along with the Cobray Terminator and the Springfield Armory Loaded M1A in 6.5 Creedmoor.

        • “Nambu Type 100 SMG”
          Not, that is simply example of too little too late rather than flawed design. It used relatively weak 8×22 cartridge, as it was Japanese default handgun cartridge, but cycled properly, so far I know.

    • “(…)Thompson M1921 with 100 round drum and pistol fore grip(…)”
      Lets see, empty weight 4,69 kg according to https://modernfirearms.net/en/submachine-guns/u-s-a-submachine-guns/thompson-1921-tommy-gun-eng/ loaded 100-rnd magazine weight according https://www.auto-ordnance.com/a-thompson-for-everyone/ after converting to human-readable unit is 3,7421 kg, so overall it is 8,4321 kg. Really heavy for sub-machine gun, but still manageable, on par with later variants of BAR, less than Chauchat, on plus side Thompson weapon is shorter.

      “(…)Any double-barrel bolt-action nitro-express rifle(…)”
      Keep in mind that there existed great variation of Nitro Express cartridges, not only such big ones as .600, but also for example .240 APEX Belted Nitro Express Holland & Holland http://municion.org/Holland/240Hh.htm ballistic-wise roughly on par with .243 Winchester. So it could actually give lesser felt recoil than then used rifle cartridges. Bolt-action repeating means limited volume of fire, but main question how heavy it would be with two barrels?

      “(…)Belt-fed AR-10(…)”
      Depend on quality of such conversion, if it would work properly, then you would have something like HK21.

      • Armalite actually made belt-fed AR-10’s and belt-fed AR-15’s as experiments for greater ammunition capacity without carrying overly bulky magazines in a pouch. The belt would be loaded into a backpack box and fed through a flexible tube.

    • @ Cherndog: OK… I’m not too “mobile” off my bicycle, but humor me…
      Of that list, I’d take a SIG AK-53 7.5x55mm Swiss modified such that it is top feeding with off-set sights. Just don’t ask me to field strip and reassemble it blind-folded!

  11. Could it be that the safety works on turning the locking ring on the back of the striker to fixate the movement of the striker and the E-M switch is to shift from (for instance) E- Einzelfeuer (single shot) to M-Mehrfeuer (multiple shot)?

    • M = “mitraillieren” means “firing full auto”
      It is one of many words of French origin used in Swiss (German language) military terminology.

  12. I spent the whole video thinking “What…and why…and what?”
    Any SIG records out there that tell us what they were actually trying to achieve with this? Perhaps Bloke might know?

  13. I think Ian forgot the accessory that would permit the gun to function. That would be the prototype silencer with a monocore of swiss cheese. Not only would the silencer help pull the barrel forward but the monocore could be used for emergency rations if you got stuck up in the mountains surrounded by french ski bunnies.

  14. Yes, “collet”. I was reminded of the fingers of the collet bushing resizing station on a MEC Grabber that squeezes the shotshell brass down to size.

  15. This post is Forgotten Weapons to the core! Please, no more variations in 9 blow-back open bolt. Rube Goldberg was no J. M. Browning but if you have bothered to read thus far, you are a GEEK and as such, you have my respect.

  16. Ah! Finally! I’d long awaited the day Ian’d find one of these… A shame the one he got a hold of had no additional info, and apparently didn’t work. I was a bit surprised he encountered this in the UK vs., say, Neuhausen am Rheinfall Switzerland? At least there he got to use the 9mm SMGs but apparently not this monster?

    Funnily enough, the SIG promo literature for this asserted it was relatively inexpensive to manufacture?
    350 rounds-per-minute in 7.5x55mm Swiss, from a 24 round magazine vs. the six-shot K31 rifle, but with similar safety mechanisms? Given the Swiss militia-based armed forces system, I wonder if the idea was that the 9mm SMG would be done away with (Hispano-Suiza mfr. Suomi kp31 and the toggle-locked Furrer fortress-only SMG), the KE-7 and obviate any non MG42 MG, and initially see service alongside the older K31 and even G1911 rifles? Notice that the Federal Arsenal in Bern cooked up an intermediate cartridge-firing FG42, which was rejected, but SIG got the nod for the Stgw57, which was slated as a “do it all for the Swiss soldier” automatic rifle/LMG/grenade launcher…

    Thanks for this episode! Truly a Forgotten Weapon!

    • “Funnily enough, the SIG promo literature for this asserted it was relatively inexpensive to manufacture?”
      relative is key word there I guess.

  17. RESTRICTED
    Security Information
    S.I.G. AUTOMATIC RIFLE
    Source stated that he had delivered ten (10) rifles to the War Technical Service on 9 September 1952 for extended test at Wallenstadt. A few rounds were fired at the Bern Waffenfabtrik on each of these rifles and they were found to function properly. The War Technical Service accepted these then for the future tests. S.I.G. has not contracted with the War Technical Service for these rifles and has retained the right to exploit [sic, export] them in foreign countries. Source stated that his relations with the War Technical Service on the M.G. 50 (S.I.G.) machine gun were very unsatisfactory and that the War Technical Service copied several features of his weapon without permission. For this reason they intend to not get involved with the War Technical Service.

    Following is a description of the weapon:
    Automatic Carbine Neuhausen AK 52
    “The automatic carbine AK 52 is a semi-automatic as well as a full automatic weapon, i.e., it is possible to fire aimed single shots from a closed breech—as with the self-loading rifle—as well as full automatic fire—as with the submachine gun (machine pistol) or light machine gun.
    “The AK 52 has the same characteristics as an assault rifle (Sturmgewehr) but it fires normal rifle ammunition [7.5x55mm Swiss GP-11] which has a much bigger muzzle energy [sic] and a corresponding flater [sic] trajectory as the assault rifle ammunition (short cartridges) by same ammunition quality. New principles in the design made it possible to overcome the difficulties raised by the characteristics that such a weapon must have, i.e., stability while firing full automatic, low rate of fire, sufficient heat absorption capacity, but nevertheless low weight [!] and small dimensions, *simple design for handling and also manufacture*, sturdiness and accuracy.
    The weapon is composed of a barrel casing with straight bore from end to end. The barrel casing is firmly fixed to the stock which is provided with a pistol grip. A breech—which does not move—is fixed by means of a bayonet catch [sic] (for dismantling and cleaning) in the rear part of the barrel casing. The breech is composed of a breech block with firing pin, firing pin spring and extractor. The barrel is guided by cylindrical bearings in the barrel casing and is locked with the latter when the shot is fired. The barrel is surrounded by the actuator prolonged by the actuator tube on which end is fixed the muzzle gland. [sic, brake? Band?]. The closing spring, which is supported at one end by the barrel casing and at the other end by the actuator, presses the latter against a protruding shoulder of the barrel. In its turn, the barrel is pressed against the breech and thus the weapon is closed. The actuator controls also the locking between barrel and barrel casing.
    The magazine is fixed just in front of the breech under the barrel. If a shot is fired, no part moves until the bullet has left the barrel. As soon as the bullet has left the barrel, the gases press on the muzzle gland which draws the actuator tube and the actuator forward in the direction of the target. During its forward motion, the actuator first unlocks the barrel and the barrel casing, then draws along the barrel. The empty cartridge case is extracted at the same time. At the end of the forward motion, the case is ejected and a new cartridge pushed at the rear of the chamber. The rearward motion of the barrel is driven by the closing spring. The barrel is pushed over the cartridge and locked with the barrel casing by the actuator at the end of the rearward motion. The firing pin is cocked during the same motion. The following shot can be only fired when the barrel is locked with the barrel casing. The type of fire—single shot or full automatic—is selected by regulating the trigger mechanism.
    The advantages of the non-moving breech and of the forward motion of the barrel after the bullet has left it are the following:
    1. Shorter length of the weapon in closed position, the length of the barrel being normal.
    2. Good accuracy because no part moves as long as the bullet travels in the barrel.
    3. Low rate of fire because the unlocking motion begins late and because the moving parts are heavy.
    4. The displacement of the center of gravity of the weapons immediately after firing—owing to the parts moving forward—gives the weapon a better stability because the increasing gravitation moment compensates the recoil moment which makes the weapon ‘climb’.
    5. Point 3 and 4 give the weapon a better stability when firing full automatic.
    6. Good and regular repartition of the heat of the barrel to the barrel casing and actuator. The heat absorption capacity of the weapon is therefore considerable and it is possible to fire a greater amount of rounds within a short time without damaging the barrel.
    7. Simple and sturdy construction, composed of few parts. The manufacture is inexpensive since there is not much milling work to be done.
    8. Easy handling.

    Attached find brochure with above description, along with three photographs showing the weapon with breech closed, breech opened and completely disassembled. It is noted in the disassembled photograph that the spring which closes the breech is not shown. Source stated this was an oversight on the part of the photographer.
    COMMENTS. This rifle is available for examination at Neuhausen and arrangements could probably be made for testing it in the United States if there is sufficient interest. Instructions would be appreciated.
    […]

    10 Sep 1952
    SUMMARY or SID Report

    This is a report on the new S.I.G. Automatic Rifle AK … Received 22 Oct 1952 ORDGU-INTEL

    SIG
    Société Industrielle Suisse Neuhausen Chute du Rhin (Suisse)
    Automatic carbine Neuhausen AK 52

    The automatic carbine AK 52 is a semi-automatic as well as full automatic weapon, i.e. it is possible to fire aimed single shots from a closed breech—as with the self-loading rifle—as well as full automatic fire—as with the submachine gun (machine pistol) or light machine gun.
    The AK 52 has the same characteristics as an assault rifle (Sturmgewehr) but it fires normal rifle ammunition….[Redacted—largely repeats above].

    SIG’s centennial gun, the AK-53, a remarkable new development in the field of automatic ‘assault rifles.’ It may be fired full-auto or semi. Full-auto cyclic rate is 300 rounds per minute, a rate so low as to permit the use of full-power military rifle ammunition in this 10.8-lb. arm without a muzzle brake. Gases are tapped off to drive a piston to the rear compressing its spring; at the rear of its travel it picks up an actuator and starts forward, the piston and actuator unlocking and carrying forward the barrel. After extraction and ejection are completed, the barrel returns to the rear, enveloping a new round. The magazine holds 30 rounds (24?), and the carbine is 39-1/2” overall with a 23” barrel.

    New Neuhausen AK 53 Automatic carbine. Uses standard rifle cartridges. Single shot or full auto. Gas operated by piston. Weight 10.8 pounds. Barrel 23-14/ inches. Overall length 39-3/8 inches. Magazine capacity 30 rounds.

    • Thanks, I still can’t understand how the barrel locks and unlocks. Perhaps more disassembly of the Swiss engineering at the rear of the barrel would help me out.

      • I agree that it is rather unclear how the flaps operate. Inside the jacket, the annular piston compresses a spring against an internal shelf inside the jacket, but fits over the “gland” or “collet” or whatever–“the thing”–and continues to drive down and compress that spring too. At a certain point, the actuator is tripped, by which time both springs are under maximum tension, and the locking flaps are unlocked… Probably a bit like straight pull, albeit the Schmidt-Rubin twists and cams the lugs out of engagement like an automatic screwdriver. More detailed photos of the area around the rear chamber area of the barrel and the connecting bits to the fixed breech might give a clue. Note, however, that the barrel remains locked and motionless until the bullet has left the barrel and the pressure has already begun to drop. Contra the speculation of dispersion, the barrel is immobile at the moment of firing and long after.

        After the actuator is tripped, the main spring under tension inside the jacket propels the captive barrel with its compressed return spring forward, along with the annular piston. At the final motion forward, the empty is extracted from the chamber by remaining on the breech face, and the barrel pulling forward off of it, and it is then ejected. I’m equally hazy about how that unfolds as about the flaps unlocking… At any rate, it then trips the “thing” such that the compressed barrel return spring pushes the opposite direction, returning the barrel to battery, over-riding the top cartridge in the magazine and somehow or other locking the flaps into the receiver. No wonder the cyclic rate was like 300 rounds? It’d be a hoot to be able to fire one of these crazy utterly counterintuitive forward-operating gas-operated automatische karabiner that is certain!

        • After the first viewing, I thought along the same lines. I couldn’t find the trip to unlock the flaps, but the piece the flaps are attached to can move back and forth without a trip. Before I get to 189 views 😉 , I am pretty sure more info is needed.

  18. AK53–Automat Karabine
    The next development [in Swiss/SIG rifle design] was in 1953 and was called the Automat Karabine 53 or AK53 of which some fifty were made for [CH] Government trial. This weapon was truly remarkable as it featured a system that saw little or no use and had no success. This is the blow-forward system [sic], in this particular application with a breech lock. In this system, as the name indicates, the barrel is blown forward by the firing of the cartridge [sic, no, not really? the barrel is unlocked, prized loose, and sent hurling forward by a compressed spring, which has been acted on by an annular pistol driven back by a gas port and the propellant gases of the just-fired cartridge, yes?] The has piston drives the barrel forward, assisted by a spring leaving the empty case to be ejected by an ejector attached to the barrel. The barrel is returned by a second spring stripping a fresh round from the box magazine and chambering it. This system allowed for a very low rate of fire, some 300 rpm and therefore no need for a single shot capability, as with a little practice the trigger could be used to fire single rounds. This weapon is chambered for the Swiss 7.5mm Model 11 cartridge as was the SK46.
    Source: A.J.R. Cormack, Small Arms Profile 10: The SIG Service Rifle [Swiss IG Automat Karabin.pdf]

    A forgotten weapon that appears long overdue for additional research!
    Too weird for its own good, it would seem… It must have equally baffled to testing commission. The version our intrepid Ian got to fiddle with apparently was the example from the Enfield Pattern Room.

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