Balanced Recoil AK-107 / Kalashnikov SR-1: Is It Any Good?

One of the really interesting variations on the AK to come out of Russian military development and testing is the balanced-recoil system, as exemplified on the military AK-107 and the commercial Kalashnikov SR-1 rifles. Contrary to common assumption, this is not a system to counteract to recoil created by the bullet leaving the barrel. Instead, it is intended to counteract the movement of the rifle caused by the bolt slamming into he rear of the receiver and into the chamber. These two events are what cause most of the movement in a 5.56mm or 5.45mm AK, not the minimal recoil of the actual bullet.

The AK-107 is the military iteration of this system, and the SR-1 is the semiauto competition rifle version, and they both use the identical counter-balanced mechanism. Basically, the gas port vents both forward and backward, pushing on two separate pistons. One moved backwards, connected to the bolt carrier, and the other moves forwards to balance the momentum of the other. The two are connected by a sort of trolley with two gear wheels which ensure the two pistons remain synchronized. This system is relatively complex to disassemble, and the Russian military opted not to adopt it.

Kalashnikov Concern then released a semiauto version called the SR-1, specifically tailored to the IPSC-style rifle competition market. It uses a plastic lower and AR-type magazines, and has a crossbolt type safety instead of the standard AK safety. It also has a very effective three-port muzzle brake. In practice, it is actually this muzzle brake which contributes the most to the flat-shooting nature of the SR-1. The counter-balanced system does work as intended, but the improvement it provides is really on par with competition-style improvements made to AR rifles – and at the cost of significantly increased weight and complexity.

The SR-1 was a commercial flop for Kalashnikov, with only one production run being made, and subject to substantial QC problems. The rifles have been discontinued, and only a small number got out into the European civilian market.

50 Comments

  1. I always had great interest and hopes for the AK-107 after watching Larry Vickers cover it years ago in Russia. Thank you for the updates to that story Ian. Sorry to hear it won’t make it to the U.S.

  2. Yann explained that the system was not designed to eliminate the recoil, but at the range he only talked about recoil.

    • He sayd that the muzzle rise is there, and it seems so. It’s hard to judge from a clip, but the rifle, without the muzzle brake/compensator, doesnt’ seem to move less than a conventional .223 Rem. rifle of comparable weight, IE an AR70 (or even better a CETME-L. Delayed blowbacks are notorious felt-recoil reducers).

  3. “(…)Basically, the gas port vents both forward and backward, pushing on two separate pistons. One moved backwards, connected to the bolt carrier, and the other moves forwards to balance the momentum of the other. The two are connected by a sort of trolley with two gear wheels which ensure the two pistons remain synchronized.(…)”
    For visual presentation see video available in
    https://modernfirearms.net/en/civilian-rifles/russia-civilian-rifles/sajga-mk-107-eng/

  4. “An AR with a lightweight bolt carrier…”

    ^^^ I suspect that that is the crux of the whole idea

    At the centre of the AK, you have the relatively heavy bolt carrier

    It is that heavy bolt carrier that is a huge part of the reliability of the AK

    It provides the momentum to extract from a hot dirty and potentially (if the chrome ever goes) corroded chamber,

    It provides the momentum to complete the rearward travel and to chamber a potentially dirty, corroded, mis shapen next round.

    That weight also allows a relatively longer cycle time, and therefore more time for the magazine spring to get the top round into position.

    These are the same arguments that I keep repeating in comparisons between pistols with browning type combined slide and breech bolt, compared to pistols like the lahti and mauser C96 with a separate light weight bolt.

    The downside of the heavy AK breech bolt is the disturbance that such a significant proportion of the weight of the gun, causes when it is slamming back and forth.

    What Ian realises, but isn’t communicating very well (sorry Ian)

    Is the mechanism is a way to keep the effective weight and momentum of the heavy bolt carrier, for reliability

    But to cancel out the disturbance that the previous one piece bolt carrier caused.

  5. The little gears and the racks are interesting.

    I think that The description of synchronicity is slightly off target.

    The gears are linking the two masses of the bolt carrier.

    Both of those racks are part of the mass of the bolt carrier

    Their momentum takes the bolt carrier through its cycle.

    I’m going to have to get a better look at the tooth profile on the gears

    And also any taper on the holes in the racks

    I’d love to see how the gear teeth were generated. I can visualise how I would approach the job of generating the involution form on say a vertical mill.

    The idea of the circular holes in the racks is excellent, no sharp corners as stress concentrators and dirt gets pushed out

    It’s also neat that two gears are used, and spaced so that they’re 1/2 a tooth out of phase with each other

  6. A person with name Collimatrix has offered his view of rifle dynamics here https://sturgeonshouse.ipbhost.com/topic/753-bolt-carrierbolt-mass-ratio/

    According to what he wrote the ratio of AK bolt carrier (I assume including piston) and bolt is little over 6 (he entered 5 which seems to be wrong). In some other sources I remember reading 7. This is a base of AK system reliability but also “liveliness”.

    Next issue is whether adding a sort of balancing mechanism into it makes a practical sense. Not according to my opinion. This sample seems to have a smaller than standard piston diameter (not sure) which by itself should help. The standard size for AK47 and even for AK74 pistons is fourteen millimeters. The SKS piston is thirteen and the gun is very reliable. And I am not talking about gas port size which is at 3-4mm. This is what people call as “over-gassed”.

    If someone (let’s say Palmetto Armory) wanted to make really a more stable AK rifle it can be possibly done by reducing the piston size to 11-12mm and gas port to 2.5mm. My feel is it would work just fine. You would immediately sense a difference and it would be still reliable. No additional hardware is necessary.

    Why it is not at standard issue AK? because Russian army easy takes the trad-off as it is. They get jittery gun but utterly reliable in all conceivable conditions.

    • It’s very interesting. Especially considered the same Jim Sullivan stated it was “crazy” to lighten the AR15 bolt carrier, and it should have been weightened instead. However it’s interesting to note that rifles usually considered very reliable violate the “at least threee times the weight of the bolt” rule regardless of the action. IE The M1 Garand (long stroke gas piston), the MAS49 (direct impingment) and the ARX100 (short stroke gas piston. Maybe that in the ARX 100 the weight of the piston had not been considered, but the total mass would be very low anyway).
      In the MAS49 slow-mo yoi can really see the bolt carrier slowing down the moment it rises the bolt.

      • Good observations dog-attendant 🙂

        The ARX has bolt group weight reduced because the gas piston is a separate unit which used to be called “tappet”. The AR mechanism works for most part by accident (mainly because gas pressure is available instantly); in my mind it will not stand a chance in dusty, dirty environment just as in deep freeze.

        Recent observation from Ukraine: soldiers are constantly covered by layer of dust created by shells explosions. They clean their small arms 5-7x a day yet guns still jam. They are AK or PK types for most part. Imagine M4 in that situation.

        • Military Arms Channel begs to differ on the ARX in deep freeze. It shown to be the best in those conditions. https://youtu.be/CVe7d7xJdAo 😉
          Obviously a single test is not a statistic, but eventual problems linked to the action should have emerged.

          As explained in the post, the weight of the piston (intending the moving part. in the case of the ARX is a mobile cylinder), even in short strokes, must be considered as part of the bolt carrier if the piston is still linked to the bolt carrier the moment the carrier starts pulling the bolt (because in that case the inertia of the piston is pushing the carrier anyway). That happens in the FAL, and in the ARX too.

          • There is little misunderstanding. I did not mean ARX100 is prone to malfunction in extreme conditions; I meant AR15/ M6/M4.
            The AR was designed by man who has a line of designs behind him; he was involved as chief designer with Steyr ACR among them.

            On mentioned short stroke piston operating mechanism the piston mass cannot be considered as part of primary mass because it acts only for very short time – it does NOT participate thru the unlocking process. It only provides the initial impetus. This is a “shortcoming” to many new rifle designs. But this is a nice way of doing it because the piston and barrel forms one replaceable unit.

          • “The AR was designed by man who …”
            —————-

            should be: The ARX was designed by man who ….

          • If the piston (intending the moving part, whatever shape it has) is still in contact with the bolt carrier the moment the bolt carrier starts pulling the bolt, then it’s part of the mass of the bolt carrier. It doesn’t count that it stops immediately after, because, at that point, the momentum of the piston has already contributed to accelerate the bolt.

          • I propose to you that it does not. Why? Because it is the MOMENTUM which it carries is extremely shortly lived.

            Think about it in this way: Impulse > momentum > impulse; they are mutually convertible at an instant and vanish just as fast. For mass of piston to be able to carry on its momentum, must be in solid connection with the rest of the action. If it is not, it does not count.

          • I propose to you that it does. Why? Because “extremely shortly lived” has no meaning at all.
            What it counts is if the piston (intending the moving part, whatever shape it has) is still in contact with the bolt carrier the moment the bolt carrier starts pulling the bolt. If it does, then the piston transfer its MOMENTUM (its mass for its speed, that’s the same of the bolt carrier, so the mass is simply added to that of the bolt carrier) to the bolt. In the case of the FAL, like that of the ARX, it does.
            THINK OF A NEWTON CRADLE. how “short lived” is the momentum of the balls that don’t even move? Yet it’s transfered.

  7. Couple of questions from an utterly ignorant yet curious cat:

    1. Does it really need the forward gas port and second piston? Wouldn’t the upper mass move anyway with the gears? Or would that sudden yank on the gear lead to too much wear and tear on the teeth and the sprocket holes?

    2. Seems to me a fair test needs to run the rifle with the counter balance weight removed and the forward piston blocked, with or without the compensator.

    • Timing. Thing is, you add more weight to the carrier in the form of the other piston without giving the other piston gas, it slows the action down considerably, possibly leading to an unreliable or even non-functional rifle. You’d either need to widen the gas port a lot… Or just give the other piston a space in the gas block, like shown in this rifle.

      As to testing it without the counterbalance mechanism, you’d probably need a way to stop up the other side of the gas block, and even then, the gas port is probably a little bigger to handle the extra weight. What happens when you take that weight off? Energy’s gotta go somewhere, mainly all in on the main piston, causing it to move faster, meaning more wear, maybe even unsafely.

    • 1. The gears and holes would be very stressed. At any shot they should suddenly accelerate all that mass. Their life would be very short.

      2. The counter balance is not there only to compensate. It’s part of the mass of the bolt carrier AND it’s pushed back by half of the total force of the recoil spring/s. By removing it you would have a lighter bolt carrier than what had been designed, pushed by the gas differently than what had ben designed, and then closed by a spring that has only half the strenght than what had been designed.

      • I don’t know whether perhaps the gears are considered to be wrar parts that get changed at intervals?

        Or are designed to last the full life of the other major components of the rifle, such as the trunnion, receiver and the bolt and bolt carrier?

        In wider engineering, gears do get used in situations where they are subject to shock loads in both directions

        A very nasty example would be in the reduction drives between piston aero engines and propellers or rotor blades.

        4 stroke Piston engines with fewer than 9 cylinders, have sufficient variation in crank speed within a single revolution, and a prop or rotor blades have sufficient momentum, that the gear teeth are suffering from repeated backwards and forwards shock loads.

        Traditional piston aero engines for light planes, such as the lycomings and continentals, were designed to run up to about 2,500 to 3000 revs, which was sufficient to avoid having the tips of the property going supersonic (think of the appalling amount of noise a harvard trainer makes with its supersonic prop tips), but without needing a reduction gear box.

        More recent small engines like the rotax 912 and 914, and the rotax two strokes, achieve a higher power to weight ratio by revving faster and using a reduction gear box

        Rotax appears to get around the reliability problem by designing a gear box that would handle many times the power that the little engine could put through it – if the loading was smooth and continuous…

        And that extra strength, resists those stress reversals and shock loads

        • Consider that all the stressed gears used in automotive are oil bathed. The pressure the surfaces of the theet are subject to is designed to not be enough to break the oil film between the metal surfaces, otherwise the survival of the gears is measured in minutes.

          • “Measured in minutes, or, seconds”

            In automotive and even in aero engines, there’s an expectation of a lot longer service life (in terms of cycles) than with rifles and machine guns.

            As an example, let’s say that the design life of a gun before it needs a major overhaul is 10,000 rounds

            A car engine cruising along at between 2000 and 3000 RPM, will achieve the same number of cycles in between 3 and 5 minutes

            Design life of even the nastiest examples of 1970s and early 80s crap cars from British Leyland or from detroit, was still 1,000 hours and more.

            Example of that was the British mini metro, with a design life of 1,000 hours / 50,000 miles at 50 MPH.

            A Decent car or a diesel commercial vehicle, the time before major overhaul / rebuild, is more like 6,000 hours.

            I don’t know whether the gears are considered wear parts, that are exchanged frequently

            Or whether they are expected to last as long as the barrel

            Either way, so long as the surfaces are lightly oiled or greased, the number of rounds that a barrel is expected to last for

            Is in line with your expectation of how long automotive gears might last without lubricant

            Is in line with

          • As said, automotive gears are not lightly olied, or greased. They are bathed in heavy duty oil. Oil that has special molecules added exactly to make the oil film harder to break under extreme pressure.
            Also the shape is different (and it took many decades to find the correct shape for gears’ theet). They are not pins into round holes obtained piercing a light sheet of metal.
            So will the timing gears and racks of the AK107 endure? Likely yes, if they do the job they are designed to do, that of the timing gears, without any significative momentum being transfered by the gears, since both the part of the bolt carrier are first pushed by the gas and then pushed back by the recoil spring.

            But the question was:
            “Does it really need the forward gas port and second piston? Wouldn’t the upper mass move anyway with the gears? Or would that sudden yank on the gear lead to too much wear and tear on the teeth and the sprocket holes?”

            And the answer is that, if those gears have to transfer a significative momentum, like for suddenly accelerating the counter-mass at any shot, they are not likely to survive for long, becaue that’s NOT what they had been designed for. OTHER gears, with OTHER shapes, DIFFERENTLY lubed, desinged to do OTHER things, doing OTHER things, is not indicative of much.

            TO ME, at first glance, in that case, the racks are likely to be damaged right at the first shot.

          • Gear tooth form first

            The most widely, and generally used gear tooth form is “involute”

            (Cycloidal tooth form is used in clocks and watches, and I think it gets used in Roots blowers as well, I’m not absolutely sure about the blowers though).

            The rack for an involute tooth form has straight line meshing faces for the teeth

            That straight line allows relatively simple, cheap to make, straight line tooling to be used to generate the involute form on circular gears of any diameter.

            That tooling can be in the form of single point shaper tools, helical hobs and helical grinding wheels.

            The actual mating face of the gear doesn’t need to be parallel to the axis of rotation of the gear

            The same applies to the mating faces on the matching rack.

            They can be helical or even herringbone.

            The circular holes and circular matching gear teeth on the AK are simply following that idea.

            Assuming that an involute tooth form is being used, then the circular holes can be formed by a tapered drill

            A rotating taper sided female cutter can generate the correct involute profile on a circular gear tooth, if a pre slit gear blank is rotated as though it was meshing with a hole in a rack.

            Involute gears formed that way will mesh correctly with the tapered circular holes in the rack and smoothly transmit movement between the racks.

            For a visual idea of the tooth generating process, just imagine that the reciprocating shaper tool is replaced by a rotating v shaped female cutter
            https://www.youtube.com/watch?v=Ppxlrgd3VW4

            This one explains the geometry of the involute tooth form

            https://www.youtube.com/watch?v=A3X8cuJKyns

          • First the shape of the gear.

            The fact that the mating face of the gear doesn’t need to be parallel to the axis of rotation (a thing that I never said), and that there is the abstract possibility for some other shape to work too, doesn’t mean that the “pins into round holes obtained piercing a light sheet of metal” will endure the transfer of significant momentum THEY HAD NOT BEEN DESIGNED FOR THAT and, to transfer significant momentum A GEAR HAS TO BE DESIGNED FOR THAT. The fact that an aircraft doesn’t strictly need wings to fly, doesn’t mean that any car can automatically fly. IT HAD NOT BEEN DESIGNED FOR THAT.

            The circular holes and circular matching gear teeth on the AK DOESN’T FOLLOW THAT IDEA AT ALL. They are holes in a thin sheet of metal. If you have examples of “pins into round holes obtained piercing a light sheet of metal” used to transfer a significant momentum, I’m eager to see them.

            AND “as said, automotive gears are not lightly olied, or greased. They are bathed in heavy duty oil. Oil that has special molecules added exactly to make the oil film harder to break under extreme pressure.”

            Not that, to me, all the oil in the world can save that rack and pinion coupling if a significant momentum is transfered. To me, in that case, the racks are likely to be damaged right at the first shot.

      • For point 1, thanks. I’m no mechanical engineer, but it’s sort of what seems right.

        For point 2, dang, hadn’t even thought of that. Yes, not something to do.

    • “Does it really need the forward Piston?”

      One of the things that you see in slow motion vid of a conventional AK being fired

      Is the barrel bending downwards as the gas system tries to accelerate that heavy bolt carrier

      It’s possible to get into all sorts of arguments over how significant that bending and subsequent vibration of the barrel is for practical accuracy achieved by the expected users.

      They’re not expected to be benchrest or Palma competitors, and they’re not expected to be firing at a nice peaceful range.

      So I’ll leave that and just say that the barrel getting twanged, probably doesn’t help.

      What the two pistons are doing, is reducing that bending of the barrel

      The combined areas of the two gas pistons will be worked out to give reliable operation.

      And with part of the mass going each way, there’s going to be a lot less bending and vibration of the barrel.

      Don’t think of the forward moving piston and rack as being somehow just a counterweight

      It’s still part of the weight of the bolt carrier

      Even though it goes forward, it is still delivering power and momentum to cycle the action.

      Perhaps next time Ian is in Slovenia, he can get us some slow motion vid and compare it with a conventional AK in 5.56×45?

      • “Don’t think of the forward moving piston and rack as being somehow just a counterweight. It’s still part of the weight of the bolt carrier.”
        ———————————————-

        Yes, true. It provides for a smoother run. You can include it into your analogy of multi-cylinder engines. Thus, perhaps not as useless as I thought of it beforehand.

  8. There was a spring piston air rifle custom built Wyscombe I believe that used 2 piston going towards each other to compress the air and negate recoil. This is because most spring piston air rifles function like open bolt submachine guns with a heavy/piston/bolt going forward to compress the air. To conteract this the artillery hold was invented basicly you do not hold the rifle tightly but let it recoil freely as in the spring piston air rifle there is little or no backward recoil not feasable on a fire arm

    • Situation are exactly opposite, 1900s French military did not accepted off-the-shelf solution in form of Hotchkiss system, as it could not be kept secret enough, AK-107 was off-spring of unsuccessful entrant to Abakan competition and https://modernfirearms.net/en/assault-rifles/russia-assault-rifles/ak-107108-eng/
      Despite the failure of both designs in the army trials, development was continued, with intention to produce weapons superior (in full automatic fire mode) to standard AK-74 for domestic police use and export.

  9. Not sure that the gears are necessary, by simply connecting functionally the two piston systems by a spring one can remove the two recoil springs and makes the bolt isolated from the receiver.

  10. Well, that was an interesting “Engineering” sort of thread, to me; knowing little about it. However, if you did this with the forward (Parts) weighing the same as the rearward parts, albeit in a bigger gun… Would that work, work better? Better at controlling recoil, so say in .50 bmg the mounts could be less substantial. Perhaps. Or not. Head scratch. I mean does it work, but like… Was put into the wrong gun, because it would need to be bigger, to work. Type thing.

    • Maybe even as a mount, you set “this” up… (The counter lark) and attach a whole mk108 cannon underneath and the, above is: Said counter lark that weighs the same as the mk108. The mk108 fires and the mount, sends the entire gun backwards and forwards… Not saying that is usefull, or otherwise; but just in relation to, the idea with the forward mass being equal to the rearward mass.

      Type thing.

      • Know what I mean, just imagine an entire mk108 as the bolt. And then the other bit; that counters I.e Moves forward, is the same weight. So yes it is twice as heavy, but… Maybe takes less screws or something, to hold it in place possibly… Mount wise… No? Less recoil, like…

        • Given an mk108 does not vent rearwards like a Rpg or etc… You would think there would be some point to it. Regardless of in 5.56mm with muzzle breaks.

        • Glue it to a bit of fibreglass; due to the mount. Cheap plane like, cheap planes are the future of war in my opinion. Due to there actually being one, be a spam war, stens etc in no time; everyone has the same multimillion dollar stuff.

          But to quote Big Mike T; everyone has a plan until someone smacks you in the mush. So to speak. I doubt these carriers /planes would last 6 months.

          • “(…)Big Mike T; everyone has a plan until someone smacks you in the mush.(…)”
            This sounds as corrupted and misfit quote from Moltke
            Kein Operationsplan reicht mit einiger Sicherheit über das erste Zusammentreffen mit der feindlichen Hauptmacht hinaus.

    • “(…) would need to be bigger(…)”
      Then comprehend Savin-Norov aerial machine gun chambered in 7,62×54 R cartridge, see 1st image from bottom
      http://airwar.ru/weapon/guns/sn.html
      observe that moveable barrel is connected via gear to rest of moving parts, thus they move in opposite direction. This weapon provide Rate-of-Fire equal 3000 rpm, which was outstanding for 1939.

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