How Does it Work: Blowback Action

The simplest for of firearms action is blowback, also called simple blowback. It is basically just an application of Newton’s 3rd Law; that for every action there is an equal and opposite reaction. As the bullet moves forward down the barrel, the slide or bolt of the gun moves backwards. The two move with the same energy, meaning that the light and very fast bullet is balanced out by the heavy and slow bolt or slide.

35 Comments

  1. Pure blow-back is generally restricted to pistols, sub-machine guns, and “pistol strength” rifles like the Winchester 1907. I hope nobody tried making a pure blow-back weapon for a serious rifle cartridge. When I say “pure,” I mean “no delay mechanism or buffer apart from the recoil spring.” Did I mess up?

  2. Greetings!

    A few years ago stumbled upon this “gem” when I was researching details on primitive gun designs (for my artwork).

    On the link you can see some open-bolt/blowback zip gun, inlcuding a 12.7mm (50cal) rifle made in South America.

    https://www.thefirearmblog.com/blog/2014/09/15/drug-cartel-diy-open-bolt-automatic-rifles/

    Im glad that these videos are being made, maybe I will be able to determine what characteristics should a “survival gun/rifle” have.

    Thank you!

  3. Cherndog: I hope nobody tried making a pure blow-back weapon for a serious rifle cartridge.

    Well, most intermediate & full-power ctgs are bottleneck, which adds a whole new challenge for blowback, and similarly with materially tapered ctgs. Can be done, but most blowbacks are straight-wall, or nearly so.

    But regardless of ctg shape, at some point, the reciprocating mass, spring forces or complex pneumatics/hydraulics result in an impractical individual weapon.

    • Shouldered cartridge does not make a noticeable difference. All it needs is restrain/ obstacle in front to cycle the action. The frontal area created by bore is what counts.

      One notoriously known example is use of training (blank) rosette type ammunition. In order to obtain sufficient operating pressure, additional ‘obstacle’ is temporarily mounted onto the muzzle. Typical name for it is “blank firing attachment”.

      • There should be not problem with cartridge caused by fact of being bottle-necked alone. For example see PPSh, so far I know it was not inclined to damage cases.
        Anyway, it is worth noting that whole point of blow-back operation is preventing too fast exit of case from chamber. That how far case might go back with still significant pressure in barrel depend on case design – there I want point 9 x 24 mm FAR
        http://cartridgecollectors.org/cmo/cmo08jul.htm
        which use peculiar design (c.f. cut-away) to allow relatively long travel back without risk of case rupture due to pressure.

  4. Needles to say, even if cartridge did not contain bullet (had a wad or crimp in front) there would still be a blowback. The reason is sudden spike of pressure acting on slide/bolt face.

      • There is whole bunch of misconception in public interpretation. I am not saying the bullet does not have a role; it does – it is kind of floating plug maintaining pressure thru the cycle. Pressure times bore section creates Force which in turn together with time period causes impulse. Impulse of pressure is equivalent to momentum of moving mass. That’s how we get ‘personal’ with it.

        I encourage everyone to look closely at p-v diagram of a cartridge. What you see is that at instant of primer initiation and shortly afterwards (in about 0.2-0.3m/s) there is a slight ramping up, but nothing is happening; bullet did not start to move yet as pressure is rapidly rising right afterwards. So, the prime function of bullet (while still in bore) is to provide means of pressure rise.

        Now one ‘funny’ detail. Some may not even take to count that to push bullet thru the bore requires substantial amount of energy. That is needed to “reform” the jacket to fill grooves. If you want to think about it form this vantage point, you suddenly realize that bullet is in fact “pulling” barrel forward 🙂

        However, once the bullet leaves the muzzle, here is another component and that is “jet effect” of gasses which is basically same as at rocket’s nozzle. This is major source of weapon’s felt recoil. For that reason short barreled weapons (of same caliber) kick more.

        • I said p-v diagram… it was intended p-t (pressure-time).
          The p-v (pressure-volume) is from another department of my head – combustion engines. They have some similarity though. I’ve heard quite fitting definition for rifle: it is a combustion engine with disposable piston! 🙂

      • What I said in previous is in no way in dispute with what Ian said. He described the final outcome of what had previously transpired inside of barrel. From this viewpoint I talk about “internal” ballistics; what he pointed out is “transient” stage e.i. bullet is just parting from muzzle.

        I merely wanted to draw attention to what is happening before projectile imparts its momentum into weapon. It is a cause and effect relationship. The start is in pressure buildup.

  5. As a first approximation where the velocity multiplied by the mass of the bullet equals the velocity multiplied by the mass of the slide {except in the opposite direction} is properly an application of “Conservation of Momentum”. Sorry for the quibble.

    • You beat me to it with the quibble 🙂

      Momentum of the bullet (mass of bullet multiplied by velocity of the bullet) is equal to the monentum of the bolt or slide (mass of bolt multiplied by velocity of the bolt)

      The distribution of kinetic energy is very asymmetric between the bullet, which gets the lions share, and the bolt.

      KE = 1/2 mass x velocity squared.

      The velocity squared bit naturally results in KE figures that look and sound impressive

      That seems to have gained some traction before about 1900, for marketing purposes.

      That probably explains a lot of the false kudos given to the 150gr .30-06 load.

      And the marketing hype given to light bulleted American loads that are only marginally effective on deer, since then, and continuing to the present day. Seasoned African and European hunters regard American loads as generally over hyped under performers ( the technical term used is “American bullshit”).

      In military terms, Hatchers notebook explains the disappointment of the American expeditionary force in wwi when their .303 and 8mm balle D machine guns got replaced with 30-06

      And the range at which an indirect barrage could be laid down, decreased dramatically

      Both .303 mkvii and lebel balle D, have lower muzzle energy but markedly better long range performance and absolute range than 30-06 150gr. Due to heavier bullets with better ballistic coefficients.

      Indeed, 8mm lebel balle d, overtakes and outperforms. 30-06 in all criteria,including ke from about 100m down range.

      Sorry for the tangent.

      • “That seems to have gained some traction before about 1900, for marketing purposes.

        That probably explains a lot of the false kudos given to the 150gr .30-06 load.”
        This lead to question: they were more concerned with energy or velocity?
        Note .250-3000 Savage with promised velocity of 914 m/s. It was offered to public accustomed to cartridge like .45-70 or .30-30 so probably it was supposed to look like 100 times “better” than .30-30.
        But this maybe explained by, what Germans call Zeitgeist (sadly, I do not know English term for that) – note that in dawn of 20th century fascination for speed was present, just like for that list: https://en.wikipedia.org/wiki/Land_speed_record#1898%E2%80%931965_(wheel-driven)
        Also in that time rich start to run events like “travel from location X to location Y in no more than Z days/hours/minutes to get given amount of pound sterling” [or whatever then used currency was named] or “make X run in shortest time”, not only in land guise (1908 New York to Paris Race), but also aviation (Deutsch de la Meurthe prize) and naval (Blue Riband)

  6. As clear, as concise, and as well ordered as anything on the net. Thank you and well done.

    If there is a way to make your ‘Idiot’s Guide to How Guns Work’ into a modular entity on this here internet, once you’ve done a few, then folk will be watching it as long as yon internet survives.

    I have fired only a very few guns. I reckon I know loads. Yet this, and your ‘open and closed bolt’, taught me things I would never have learned (and because of that ‘Become a Forgotten Weapons Patron’ has moved higher up my personal league table).

  7. Really appreciate this series. I’m from a non gun country, the UK, living in Belgium at the moment. So my gun knowledge is limited. I had some idea of that terms like open bolt, and blow back referred to. This series really clarifies the meanings for me. Really adds to my understanding. Thanks.

  8. Energy is not momentum. Momentum is not energy.

    Momentum is measured in units like kilogram * meter / second, or kg m / s , which doesn’t have its own special name. Ignoring Einstein, you calculate an object’s momentum by multiplying its mass by its velocity. ( momentum = mass * velocity )

    Energy is measured in units like kilogram * meter * meter / second / second, or kg m^2 / s^2, which is a joule. Ignoring Einstein, you calculate an object’s kinetic energy by multiplying one half times its mass times its velocity times its velocity. (kinetic_energy = .5 * mass * velocity^2)

    Momentum is conserved, so when a thing flies into pieces, the momentums of all the pieces have to add up to zero. (What that means when there are many pieces flying around in three dimensions brings “vector math” into this. Let’s ignore it for now.)

    For the oversimplified case where a solid gun fires a solid bullet, the mass of the bullet times the velocity of the bullet plus the mass of what’s going backwards times the velocity of what’s going backwards has to sum to zero.

    This does NOT happen with energy.

    For the low velocities we’re dealing with here, kinetic energy is one half the mass times the velocity squared. Since the bullet has low mass but high velocity, it ends up with the lion’s share of the energy. This is a good thing, because if energy DID balance, the shooter would get nailed in the shoulder with as much energy as the target does.

    Concrete example: A 2 kilogram gun fires a twenty gram bullet east at 1000 meters per second. How much momentum does the bullet have, how much momentum does the gun have, and how much kinetic energy is in each?

    The bullet has (mass * velocity) = .02 kg * 1000 m / s = 20 kg m / s of momentum.

    The gun has to end up with -20 kg m / s of momentum–that is, it has to be moving west–at some velocity. We know how much it masses, so we calculate 2 kg * velocity = – 20 kg m / s. Divide both sides by 2 kg and we get the gun’s velocity = -10 m/s.

    The kinetic energy of the bullet is .5 * mass * velocity^2 = .5 * .02 kg * (1000 m/s)^2 = 10000 kg m^2/s^2. Turns out a kg m^2/s^2 is a joule, so the bullet has 10000 joules of kinetic energy.

    The kinetic energy of the gun is .5 * mass * velocity^2 = .5 * 2 kg * (-10 m / s)^2 = 100 kg m^2/s^2, or 100 joules.

    In reality, there are more than two things moving–there’s the gas, and there are internal gun parts doing various things. But the basic insight still helps explain why the guy struck by the bullet is in much more danger than the guy struck by the recoiling firearm.

    • Beyond that energies being unequal I want to note one thing which make difference even bigger – area which is used for transform of said energy, i.e. energy per square cm (or energy per square inch).
      Upper limit regarding bullet is area of circle of diameter equal to bullet
      Lower limit regarding rifle is butt-plate area
      Note that I used Upper and Lower there as generally most bullets are not cylinders, so impact area is smaller and not all energy is transformed through butt-plate, as there might be also vertical grip and some part is going through hand gripping fore-end.

  9. I like simple terms without much of undue theory (and God forbid, math) so everyone can understand, as I do. It does not need sir Isaac Newton (peace be upon him) not to mention near-to-God Herr Ein-stein. All this what we talk about exited way before those notable gentlemen lived. We humanity are so-so ‘catching up’ with nature.

    Btw. how do you think they crafted firearms at Fabbrica d’Armi Pietro Beretta in middle of 16th century…. none of those honorable names were known, yet the guns did what was expected from them 🙂
    Were they supposed to say: “hold on, no shooting, Newton was not born yet” 🙂 No way, they had to deliver to customer what he ordered!

    Regarding lady with lovely name Energy or Energie, if you wish – this is outside of weapon’s dynamics. Weapons are concerned with pressure, force and momentum. Sure, bullets yield energy… nothing to dispute there. Other than that, it is just “foot-pounds”.

    However, if you do not hold your firearm (likely rifle) you come to know what ‘energy’ is about: it will kick ya. That happens because rifle’s butt picks up its own velocity in the space you left between it and your frame.

    • Early cannoneers found their guns did not do what was expected from them. Many assumed the projectile would fly straight and exterior ballistics advanced with the introduction of Newton’s laws. Wile math may be good for those with an aptitude for it, or insomniacs :} , early computers were used to do complex long range ballistic caculations, and were a driving force behind the development of computers.

      • Yeah right…. early canoneers. I believe the most common method of learning is called “heuristic” – by previous experience. I works all the time. But, in truth (and Monsieur Newton will excuse me for second) is was I-n-t-u-i-t-i-on which brought us to where we are right now. And yes, accidents. Such was the case with rifled bore – it was discovered accidentally, actually by error during barrel making.

        As of myself I had 2 major inputs to my knowledge. One was super-easy to read notes from lectures by Mr.J. Rocha given to staff of Springfield Armory in early 60s (Ian brought it up some time back); the second was by one very knowledgeable gentleman who was working as a contractor where I was. He turned my bulb on 🙂 So far, I am reasonably comfortable with what I know, but there is genuine need to add and refresh.

        • Sure, artillery was an art, I get that and am appreciative of what they were able to do. Ian tries to be accurate, and that is Great. But if the pronunciation of Mr. Garand’s name is important as is the AK AK-47 nomenclature, which they are, calling conservation of momentum an “energy balance” just isn’t right.

          When Rollin White showed Col. Colt his revolver design, Colt didn’t tell him “you can’t break Newton’s 3rd law.” Whether he knew it or not as such, in effect that was what he was saying. 🙂

          • They likely did not care for such nonsense. Their priority was to make the design work and make money out of it.

          • “Their priority was to make the design work and make money out of it.”
            For that must have is ability to create (and understand) requirements otherwise you might need with such contraptions:
            http://enrique262.tumblr.com/post/181891251740/the-ghanaian-company-kantanka-technological-centre
            (I am still wondering if they did not borrowed these asset from some movie studio making cheap films)
            Whole numerical operations or as you wish “science” part is essential, unless you are ready to craft single new prototype every time you alter any value.

          • Daweo,
            situation on this continent is different that in Europe. People are more “hands on” (based on experience) oriented and less scientifically prepared. If they need some special work they contract those who have proven track record.

            On general note, great designers of firearms in U.S.in past were no more than mechanics/ technicians. So was J.Browning as much as was E.Stoner. Yes, they were talented people, but they were definitely not “engineers” in true sense of the word.

          • Denny: Browning and Stoner may have not known the laws of physics. But you can be assured that they didn’t break them.

  10. Due kudos to the commentors who discussed the difference between equality of conservation of momentum and the difference in kinetic energy, long before I posted my reply.

  11. Ok, onto epistemology.

    Thomas Kuhn, in his brilliant fifty something year old book; the structure of scientific resolutions, makes a number of unfortunate errors.

    He considers that no one paradigm is better or worse than any other

    And he regards engineering as the lowly practice Of applying well developed science to lowly practical problems.

    On the first point (paging neighbour J here), there are ways that result in successful results ancient ways that don’t.
    Not all paradigms are created equal.

    Sixteenthly; empiricallly, science does not lead engineering practice, it tries to play catch up.

    Ballistics, thermodynamics, optics, mechanics…

    In pretty much every field, the scientists might have had higher credentials in the social status league, but they were way behind in explaining the engineering practice.

    I’m not going to criticise the formalisation of models which can be used numerically. They’re damned useful.

    But empiricallly, the practitioners step way infront October the scientists

    Whatever the lamestream history might tell you

    History > his story (who’s story?) Wtf told you that crap?

    • That hasn’t been true for at least 84 years. Leo Szilard and Enrico Fermi patented the atomic reactor in 1934. The first one wasn’t built until 1942.

      You can still argue correctly that most of the developments in human history didn’t stem from organized science. But that’s mostly because what we describe today as organized science, complete with publication in journals, only dates back to the first journal: _Journal des sçavans_ in 1665.

      • And even in the limited field of reactors,

        In what limited parts of a very limited field, did science get there ahead of engineering – as Kuhn claims that it must?

        And if kuhn’s other (nihilistic)claim, that no paradigm is superior to any other, were true;

        What other paradigms would lead to a nuclear reactor working?

        • Let’s start with knowledge that putting together a critical mass of a fissile led to a sustainable chain reaction. This was established in theory WELL ENOUGH TO PATENT IT eight years before Fermi built an actual reactor in a squash court. And then there’s plutonium synthesis, isotopic separation, the use of moderators to slow neutrons, the use of rods of neutron-absorbers to soak up excess neutrons to turn the verdammt thing OFF, etcetera. None of these originated with the folks building the reactors; they’re all things conceived of and planned at a blackboard before anyone so much as picked up a ball of wax.

          Check out Richard Rhodes’ _The Making of the Atomic Bomb_. He goes all the way back to Planck in 1879. Read that, and you’ll have a handle on just how much academic stuff had to happen before anything remotely usable for anything came out of it.

          I don’t know where you get the notion that Kuhn equivalences old paradigms with their replacements. The entire point of a new paradigm is it opens up new things that weren’t possible before. . . . You’re not thinking of the Sokal affair instead, are you?

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