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How Does it Work: Blowback Action


Blowback is the simplest form
of firearms operating principle. And it’s really just an application of Newton’s third law: that is, the idea that for every action
there is an equal and opposite reaction. In the case of a blowback firearm, the
action of the bullet moving down the barrel is balanced by an equal and opposite
movement of a big chunk of the gun’s mechanical bits going the other direction.
Now in this case it’s a pistol slide, but for other types of blowback firearms, it can be
other parts of the gun, or parts with other names. As a firearm that loads through the breech,
or the back end of the barrel, we of course have the barrel open at both ends, and when you fire you’re creating a tremendous
amount of pressure in the barrel of the gun, many tens of thousands of
pounds per square inch of pressure. What’s critically important is that the back end not
open up while all this pressure is still in the barrel. If that happens, it will explode out the back
and potentially harm the person holding the gun. That is the job of this brass cartridge case. It is there
to maintain a gas-tight seal in the back of the barrel. Now, with a blowback firearm, as soon as
you fire, the bullet starts moving forward, and because of Newton’s law the slide
immediately starts moving backwards. What makes this able to operate safely is the
difference in mass and velocity between the slide (or the other part of the firearm that’s
blowing back in a different design), the difference between the mass and velocity
of this and the mass and velocity of the bullet. Because what we have to balance, according
to Newton, is energy [momentum]: that is, a combination of mass and velocity. The bullet has, relatively speaking,
a very low mass and a very high velocity, which means when we have a slide, or other mechanical
part of the gun here, with a much, much higher mass, it is going to have a proportionally lower velocity. The trick to balancing this so that it functions
properly, is to make sure that the slide is heavy enough that it doesn’t open enough that the brass case
might rupture before the bullet has left the barrel. Once the bullet leaves the barrel, then all of this
gas pressure just vents out the front of the gun. In the practical world, this works out to be a system
that works quite well with pistol-calibre cartridges, relatively low-power cartridges, and not
so well with higher-power rifle cartridges. And that’s all just a matter of balancing the
amount of energy going downrange in the bullet with the amount of mass that has to
reciprocate backwards in order to balance it out. So in a pistol, as we just saw, that’s the slide. In a submachine gun, that’s the bolt
operating up here, it does the same job. This acts as the mass that goes backward at the
exact same time as the bullet begins to go forward. Technically this can work with any
power of cartridge, however, once you get above large pistol cartridges, the amount of
mass required to adequately balance the bullet becomes quite large – large enough that it’s no longer
really all that practical to design a gun in this way. There are a few variations on the blowback
system that introduced delaying mechanisms, but we will cover those in a separate video. Hopefully this has answered some of your
questions about what a blowback firearm actually is. I’m Ian McCollum with Forgotten Weapons,
thanks for watching.

100 thoughts on “How Does it Work: Blowback Action

  1. I'm loving these videos for the basics of gun operation. I find the history fascinating but living in the UK the finer details of gun operation and shooting are a lot less well know and discussed. 10/10 for deconfusing Brits

  2. Okay, as a gun newbie – though pretty good at physics, I appreciate You giving us lessons on how the guns operate – yes I'm from Europe, I didn't have opportunity to study those things, apart from theory. And I prefer learning from examples.

  3. Mostly correct but it is not not energy but impulse or momentum that is conserved. The energy distribution is even more disparate, the energy distribution being the square of the speeds of the bullet and slide times their mass. Hence the bullet carries away most of the energy.

  4. Excellent series.
    Hope the series will one day cover firearms handling so that one day I won't see a photo of a cop holding homemade MAC11 with trigger zip-tied back. Hopefully no one puts a loaded mag in and runs the bolt back…… because I wouldn't trust the 3 safeties, that's assuming it even has any built in.
    https://i.cbc.ca/1.4259117.1503521115!/fileImage/httpImage/image.jpg
    Its old news from 2017 but here's a link to original article:
    https://www.cbc.ca/news/canada/edmonton/weapons-trafficking-edmonton-machine-guns-1.4258821

  5. You can do better. But I have heard in the gorn tubes that: the greater the receprecating mass the less the felt recoil. I have also heard the opposite. Which one is it?

  6. Might want to add the videos about machinegun nomenclatire in this playlist. They kinda of are the same type of video. Althought much longer.

    Great vid !

  7. Just thought I’d mention that the velocity of the slide is NOT proportional to the velocity of the bullet (based on weight). The formula for kinetic energy is 1/2 x mass x velocity^2. So, since the energy is equal, when the proportion of the slides mass to the bullets mass is taken into account, the velocity of the slide is more exponential than proportional.

  8. Love these videos and I am kind of surprised you had not started these a long time ago. For some of us, it's information we already know, but it's still valuable to the community to have it all on the same channel or site rather than saying "If you're interested in this sort of thing, check out X's video on the topic". Initially I used to think FW was just entertainment and cool guns, but I've come to appreciate the amount of education you are doing.

  9. Not sure if it was intentional, but you never mention the role of the recoil SPRING which (of course) acts in concert with the slide/bolt mass to slow the opening of the breech.

  10. I absolutely love your videos, I always thought I new what blow back action meant and was kind of there, after watching your video I know now how it totally works thank you

  11. Really excited to see more of this series, thanks for making it. I feel like I'm fairly knowledgeable about these things, but have learned something from both so far. If you're looking for ideas: delay mechanisms, sear designs, DI vs. Piston gas systems, maybe a bit on optics (paralax, focal planes, zeros related to trajectory). Thanks again!

  12. These short info videos (or would that be infomercial) are just the right touch, Ian. Not a full blown lecture, but enough to satisfy your curiosity. Thanks, Ian

  13. Question….. is there a measurable difference in muzzle velocity between blowback, delayed blowback, and say, a revolver or bolt action gun assuming same cartridge and barrel length? Just wondering if, and how different operating systems affect ballistics…

  14. Well what was that gun you showed in that clip that rifle short-barreled low back long slide I've been thinking about building something like that

  15. Not to be too pedantic, but it's momentum that's conserved, not energy. If the energy is conserved, guy behind the gun would be knocked over or worse. There's undergrad Physics experiment called ballistic pendulum that illustrates this concept. It was run with real rifle before antigunners stepped in.

  16. Blowback actions use gas pressure, not recoil. Consider that blowbacks cycle blanks if the bore is restricted. This would not work if recoil was required, but does work because gas pressure is what operates blowbacks.

  17. Excellent series! The only addition I can think of might be slow-motion cutaways and/or animations to help explain more complex machanisms.

  18. Hey maybe this channel or another commenter will know.. I have been trying to find this certain pistol that when fired, the slide moves forward, to counteract recoil. I saw one video of it years ago, and havent found it since.

  19. The 'How Does it Work' series is easily my favorite, or at least the one I find the most consistently interesting. Just because I'm not always interested in every type of gun or every book, but the mechanical principles that are getting explained keep showing up again and again.

  20. Despite loving the engineering behind guns a whole lot, I never really grokked blowback action until this video. Thank you so much!

  21. If the pistol was a bolt action (not worrying about recoil or kick )would the bullet have a higher muzzle velocity hence more power . Does the blow back take some of power and accuracy out of the pistol ? Any comments would be appreciated !

  22. Great video, but….
    HOLY GUN JESUS WHAT THE ACTUAL P*NIS HAPPENED TO THAT MAC-11!?!? WHAT IS THAT RECEIVER AND WHY?!?

  23. Thanks Ian, I was wondering why someone would go to the bother of a gas operated system even for cheap rifles like the AK47, now I know !

  24. Hey Mr Ian may i respectfully request to repost this lesson to my professional Facebook page Mark D. Aul Hobbiest Gunsmith? I love your videos and learn a great deal and respect your work so i request your permission Thanks Pls advise

  25. I love the simple explanation, I honestly never really gave it thought on how a blowback works. Though, I am just new to shooting so cut me some slack lol

  26. So there should be an mathematical formula for calculating a bolt mass for every caliber. And what mass should be for for example 9mm? Also Ian did not mentioned that closed bolt blowback actually lighter then open bolt ones

  27. 1:41
    It's not energy that has to be balanced but momentum.
    Momentum: P=ma
    Energy: Ek = ½mv²
    If you look at the equation for kinetic energy you see it is equal to half of the mass * the square of the velocity and thus when you have 2 objects with the same momentum the lighter one (the bullet) will have more kinetic energy.

  28. Gun Jesus introduces science – Newton's 3rd Law – in the first sentence of a gun video; gun community just got serious for the first time, seemingly ever.

  29. It's odd to me I thought about how it works the other day. Didn't look it up, didn't speak to anyone, didn't even say or mumble it aloud and this showed up as a suggested video.

  30. Isn't this a little tricky for open bolt weapons?

    So if the round fires just before the bolt has entirely closed (which may be by design and a desirable feature) the bolt will still have the forward momentum from being pushed forward by the spring, effectively the force on the back of the cartridge doesn't just have to accelerate the bolt backwards but also cancel out the forward motion (toward the muzzle) inertia of the bolt. However, if for whatever reason the bolt completely closes before firing (such as an extremely short hangfire or the round being fully in the chamber) then the bolt will instantly lose inertia, transferring that momentum to the weapon (which may move the weapon forward slightly, and so, open the action slightly) or may even bounce off the barrel and have a rearward velocity or open slightly at the moment of firing.

    I know open bolt MAG FED weapons are considered "Machine guns" under US law no matter what, but open bolt is not considered a machine gun if it lacks a magazine nor any other mechanical means of having another round held ready to be fed into the chamber.

  31. Pressure must also be an important factor. For lower pressure pistol rounds, the pressure is low enough that the walls of the case can slip against the chamber so if the base of the case is pushed back 1mm then the walls of the case will follow it. With higher pressure rifle rounds, if the base of the case moves back 1mm then the walls of the case will likely still stick to the walls of the chamber and the case will split. Helps that pistol cases also just have less surface area than rifle rounds.

    The higher pressure of rifle rounds means a blowback action will be pushed open even quicker, due to all this, straight blowback is so much harder with rifle cartridges.

  32. Technically, doesnt adding any "delay mechanism" to a blowback system make it a breech lock, and thus not blowback anymore? Are you referring to roller delayed systems?

  33. Im liking this series. Its cool to have those quick and easy answers, to google up and hand off to noobs as resources. I cant tell you how many tines ive had to explain a concept at a bar to an anti. Can we please cover assault rifle? I know its a rough one, especially since federal, states, and media all define it differently. But it would make my life easier.

  34. Great series. One word I hear you use often is "sporterize". I can get the meaning from context, but what specific features are you talking about here? Are there legal reasons to do this, or simply ergonomic ones?

  35. At 1:40 you make an important misunderstanding of the physics equations: you have to balance force and momentum, not energy. P=mv for momentum, but k=1/2mv^2 for energy.

    Thus the bullet carries far more enery than the slide of the gun, despite having the same momentum.

  36. It's not so much that rifle cartridges are too powerful for a true blow back design, but has everything to do with the cartridge shape itself. In fact, a 223 has roughly the same momentum, NOT kinetic energy, as a 9mm. Once a bottle necked rifle cartridge moved back even the slightest, it would no longer be supported by the chamber due to the shape, causing some serious issues like case rupture and injury to the shooter. This is not an issue with straight walled cartridges or even slightly tapered cartridges such as the 9mm. Hi Points come to mind.

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