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How Does it Work: Open Bolt vs Closed Bolt Firearms


An open bolt gun is one in which the bolt locks open,
as the name suggests, until you are ready to fire. So, in the firing condition the
gun is like this, with the bolt back. When you pull the trigger, the bolt
goes forward, picks up a cartridge, and fires it as soon as it is fully closed. The alternative to this is a closed bolt
firearm, in which the bolt is already closed when you pull the trigger. There it’s ready to fire, and then it does fire. The question of open bolt versus closed bolt
has no necessary relationship to whether a gun is fully-automatic or semi-
automatic. As well as no relationship to whether it has a locked breech
or is a simple blow-back action. We typically see open bolt guns on full-auto
firearms, machine guns, but not always. Here is an example of a fully-automatic, open
bolt gun that does have a locking breech. You’ll notice the rotating bolt there. When the
bolt closes it rotates to lock, and then it fires. As opposed to the submachine gun you saw
earlier, which is a simple blowback system. We’ll touch on the difference between
locked breech and blowback in a future video. Both open bolt and closed bolt systems
remain very common today, with open bolt guns being typically fully-automatic
machine guns in a variety of calibres. Anything from submachine guns
up to heavy machine guns. The advantage that an open bolt system
brings to a machine gun is that it does not leave a cartridge in the chamber. So, when
one is not actively firing, the next cartridge to be fired is sitting in the magazine,
and not in the actual chamber. In a fully-automatic firearm the barrel,
and thus the chamber, can get very hot, potentially to the point of being hot enough
that if you leave a cartridge sitting in there it will heat to the point that the powder,
or the primer, spontaneously combust and fire without the trigger being pulled.
That is called a cookoff, and it’s a substantial potential problem for a fully-
automatic firearm, particularly one that is belt fed. In addition, having the bolt remain open like
this does leave both ends of the barrel open and allows a little bit better airflow to help cool
the barrel, although that’s a secondary consideration. You might wonder why all firearms aren’t
open bolt designs if they have this advantage. The reason is that it is more difficult
to accurately shoot an open bolt gun, because from the moment that you pull
the trigger on an open bolt gun you have this entire mass of the bolt moving forward,
locking (if it’s a locked breech gun) and then firing. This gives a lot of
time for your aim to adjust, to slip, and it also means that you have this
impact when the bolt actually fully chambers the cartridge. That’s going to jolt the gun, and
again cause potential problems with your aim. On a closed bolt firearm like this, the bolt
is already fully forward with a cartridge chambered when you are ready to fire,
and so pulling the trigger doesn’t cause any heavy parts of the gun to move,
just the hammer or the striker. This makes it more accurate as a practical matter. In addition to the practical concerns
about shootability, there is also a factor of open bolt firearms are typically simpler,
easier and cheaper to manufacture. And so there are also a class of semi-automatic,
typically, open bolt sporting forearms produced simply because they were
less expensive, like this .22 calibre rifle. In the early 1980s the ATF in the United States
came to the determination that semi-automatic, open bolt firearms were easily
convertible into machine guns, and thus issued a ruling that semi-automatic, open bolt
guns would no longer be legal to manufacture. Guns that existed at the time of that decision,
like this one, were grandfathered in and may be owned and sold and
bought and traded in the United States, but new production of semi-automatic,
open bolt firearms are prohibited. This is not necessarily the case in the rest
of the world, as there are many countries in which semi-automatic, open bolt
firearms are treated no differently than semi-automatic, closed
bolt firearms under the law. Hopefully that has answered some of
your questions about the difference between open and closed bolt firearms operation. I’m Ian McCollum with Forgotten
Weapons. Thanks for watching.

100 thoughts on “How Does it Work: Open Bolt vs Closed Bolt Firearms

  1. You didn't answer the question, was the atf right about open bolt being easier to convert to auto. Or was it just another dumb atf rule…

  2. This is good content. The channel has historically been marketed to an audience that knows guns but knows little about odd or failed guns. It's nice to see content which can be used to educate people who are new to guns entirely. Very helpful, please continue.

  3. Offtopic – Ian always talks about these weird weapons that have these special traits and I've wondered…. Like there is Formula1 racing, there should be top tier shooting with speed/accuracy courses in 5.56, but the actual caliber is the only limit, and maybe regulated mag swaps like 3 per course or whatever. And gun manufacturers would field these "factory" sporting guns that are good for only those few seconds on the range at a time like racing was done in the golden times – complete engine rebuild after each race. Gun Formula 1. 😀 Those would be interesting most likely.

  4. Just love this east german forniture. Nice thing about this video is that even if I knew those things before it didn't bore me at all because of all that examples variety

  5. Ian, i do hope that gevarm is now part of your collection.
    on this side of the world, they are around but the police really discourage them due to the amount of idiots that convert them.
    bit of a shame as they are excellent guns to use for game and pest control.

  6. How is a open bolt simpler than a close bolt design? What goes into one? Is it a matter of simply a fixed firing pin on an open bolt vs a moving firing pin on a closed bolt?

  7. I hope you do all the different type of recoil and gas systems next 👌🏻👍🏻 That would be an amazingly informative video and I think very beneficial to may people

  8. Thank you for the explanation that open bolts can’t be manufactured anymore. When I was looking at FN’s M249 repo I was thinking of dropping the money on it but realized it was closed bolt as oppose to the open bolt. I couldn’t find out any reason behind this and the only thing I can gather is they did it for reliability. That is until I stumbled upon this video and now fully understand. I’m still on the fence of getting the civilian m249 as it would be cool but too bulky and heavy to be useful other than a talking piece.

  9. I guess i had figured most of this out in my head and already had the accuracy problem with an open bolt and never thought of cookoff being a reason to have an open bolt action.

  10. Oh my god, yes! I love this idea. If you ever decide to make a series on theory like this, I would gladly pay for it!

  11. Thank You! I always hear these terms, and then spend 30 minutes google searching answers, lol. Your <5 minute video was so much more simple & informative. Love your channel!

  12. Could you do a video explaining exactly how triggers work. You often take a gun apart and say thats the fire control group but I cant find very much info on exactly how they work. One question why do they use a sear. It seams the one part that as it wears would make a gun more dangerous. Why dont they use a system that with wear makes the trigger harder to pull such as a toggle lock that goes further over center the more its worn out.

  13. I like this format a lot. When I first stumbled onto your channel I didn't know anything about the different types actions, (blowback, delayed blowback, long recoil, etc) I slowly picked up the information through context but it would have been great to have a reference like this to learn a lot of the basics you use when describing weapons.

  14. Would it be possible to make a playlist on your channel for the "How does it work" series? Thanks for a really interesting and informative video. Be great to see more!

  15. Would that also explain why some automatic firearms have their bolt repeatedly knocked back with each shot while others don't?

  16. Do open bolt not pose a large(r) risk of dirt getting into the action and barrel? I'm always cringing about that when I see an un-shrouded open bolt gun, especially for rougher application like warfare etc. I've never owned an open bolt gun, but this is very interesting

  17. Query from a Briton with no guns. What's all this about the old Bren gun being used for the longer ranged sniping? If it's open bolt, then surely it won't snipe very well, no?

  18. This video made me to look more into your channel. Can you make a "Basic Explained" playlist/series so every newbie can get more interactive within technical side of the terminologies. Once agai, Great video!

  19. These videos are very useful for people who live in countries where guns are not very common (Australia). Although i could go look this up on google, this video explained it clearly and concisely. Great video, very informative.

  20. I finally understand the different reason behind both designs, and on top of that, I understand why FG42 has both mode at the same time, from a 4 min video. Great one, thanks!

  21. Damn, bless this channel. I don't even have any firearms, I'm just a big enthusiast, and I love these explanations.

  22. Excellent video. Question – are you aware of any weapons which fire from an open bolt in full-auto mode but a closed bolt in single shot mode?

  23. I love this format. Short, digesable information that hits the right spot between overly simplified and too technical

  24. When I saw this video on my feed this morning my first thought was "I wonder he'll explain locked breach vs. blowback?" and lo, you mention it first thing 🙂 I hope you touch on straight blowback vs. delayed!

  25. Out of curiosity – is the BATFE decision that said open bolt semi-autos are too easily converted to FA easily available anywhere?
    Thanks for all your hard work and fantastic presentations!

  26. very clear. well done. i am now begining to tell anyone i talk with about firearms engineering to watch your vids. thanks again

  27. Thanks for this. It’s good to have a short quick and reasonably detailed explanation of what the difference is for people who don’t know guns.

  28. Before watching this video I knew precisely 0 things about how guns work. Now I know precisely 1 thing about how guns work.

  29. Really awesome video. Extremely thorough and well-explained. I'll have to refer this to anyone who asks me the same question.

    Should've shown the FG42 in the end just as a little mind-blower 😉

    Very informative educational video nonetheless. Love your work Ian. Keep it up!

  30. In Germany open bolt guns are generally considered as "weapons of war" and it is strictly forbidden to legally own them as a private person. There are for example semi auto versions of the Uzi for sale (can be bought with a valid hunting license) but they have to be converted to closed bolt. As always, very good video!

  31. You forgott to mention two advantages for closed bolt systems.

    -accidental shooting is less likelly to occure.
    (For example You can shoot an open bolt if you charge it inproperly)

    You can seal the gun. Mud does not come in.

  32. Thanks Ian for clearing up things. There is a lot of terminology that i kinda sortha understand, and videos like this make it easyer for me to follow.
    Happy Hollydays from Croatia 🎅

  33. My question is what open bolt semi rifles where being converted and how did they come to that determination?

  34. Open bolt: one does not simply chamber each round and remove the cartridge manually
    Closed bolt: one does not simply spray and pray at a target.

  35. I've been a shooter all my life but I have not fired a wide variety firearms. I only know a bit about the ones I've used. It's hard for an uneducated person like myself to assess someone more educated, but this gentleman is well-spoken, to the point, and speaks with confidence. He makes me want to learn more from him.

  36. One of the "benefits" of the open bolt design that i have heard talked about is that the chance of a "cook off" is essentially eliminated. Is this a feature of the open bolt that was intentionally designed for, or was it a happy accident?

  37. As far as closed bolt designs go, there is also the advantage of minimizing the dirt and grime that is capable of finding its way into your gun.

  38. One question: I have heard that closed-bolt design is (or at least was) necessary for weapons being fired with an interrupter gear through a propeller. It allowed for more precise (on the scale of milliseconds) timing firing of each round. Is this true?

    I was reading a history of the industrial side of the US war effort, and this was the reason given for focusing on the existing M1919 rather than developing an open bolt alternative; the gun could be used on fighter planes as well as by infantry. This choice was in 1939/1940 before the lessons of the Battle of Britain and the decision had been made to switch to .50 cals as a minimum for fighter armament. Is there truth to this or is this what you get when an economic historian writes about weapons?

  39. so basically the batf created a law again for something that was not a problem but for something that could possibly be done illegally. if it is already illegal to possess or modify a firearm that is full auto without proper paper work and registration then this is just another redundant law to infringe with no actual purpose. cause all semi auto's can be converted, open or closed bolt.

  40. That gun's barrel reminds me of the mgd pm9, which reminds me of how awesome it would be if you made a video firing it.

    But I understand that you probably can't, it's just I can't find any other videos on it.

  41. Guns in this video:
    0:00 MAS 38
    0:14 M3A1 "Grease Gun"
    0:21 Hotchkiss Universal SMG
    0:33 Chauchat
    1:23 Colt R75A BAR
    2:53 AK (to be specific, that's an East German MPi-KM-72)
    3:11 Gevarm A6

    Videos on the weapons:

    MAS 38
    https://www.youtube.com/watch?v=xOX21MhEIXo

    M3 Grease Gun
    https://www.youtube.com/watch?v=6ivr4QdhVtU

    Hotchkiss USMG
    https://www.youtube.com/watch?time_continue=222&v=pRrFWQXl-RE

    Chauchat
    https://www.youtube.com/watch?v=bCwP3Dm52Ls&vl=en

    BAR R75A
    https://www.youtube.com/watch?v=BxIOdnoPnxM

    AK (Any AK video will do, so here's videos on the very first iterations of the OG AK-47)
    https://www.youtube.com/watch?v=zFagaHLuekQ
    https://www.youtube.com/watch?v=dL-1dauhH6M

    Gevarm A6
    https://www.youtube.com/watch?v=o6pVgM-tKA0

  42. Many hugs to Ian for clearing that up. As a dude with a sort of lukewarm interest in firearms, just enough to watch some youtube vidoes now and then, I am unaware of a lot of basic concepts, and this explained it very well.

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