How Does it Work: Open Bolt vs Closed Bolt Firearms

This is the first of a series I am doing on specific different types of firearm action and operation principles and systems. I want to keep them short and to the point, to create an easily referenced library for folks who don’t know what the various terms and mechanical systems are.

Most semiauto firearms fire from a closed bolt and most fully automatic firearms fire from an open bolt, but these are far from strict rules, and many exception exist. Open bolt offers better cooling and prevents any possibility of cookoff, while closed bolt offers better practical accuracy.



      • A little more evolved than the FG42. FN HMAR switch automatically from closed bolt operation to open bolt when the chamber temperature goes over a threshold and than revert back to closed bolt when it had cooled enough. With no electronics involved nor operator action required. All is purely mechanical.

    • Thank you! I was sure I had heard of this, and was sure it was a scar, the hamr variant name is what I was missing. Hopefully Ian can get his hands on one!

  1. I hate to tell you this but the barrel on your MAS38 is bent. Or is it the receiver? Ha!

    The Hotchkiss Universal ‘origami gun’ is very cool.

  2. Here’s a thought: Why don’t we crowd-source a list of things here in the comments that are most confusing to the neophyte, or which are most prone to being misunderstood. Not to mention, the sheer myth-takes you run into, like “Mattel designed the M16”.

    I’d vote for a clear and understandable piece on headspace and timing; I swear to God, trying to get the average person to wrap their heads around that bit of arcana has been the bane of my existence as an MG trainer, particularly where it runs into “Why PFC Schmedlap should not swap bolts between guns…”. Ain’t nothing like having to ruefully call up the folks at Third Shop in the middle of a deployment, and ask if they can come headspace every rifle you have, ‘cos some certified gene-e-yous lieutenant chose to gather up every bolt on that chalk (aircraft load), and then dump them into a bag with no marking as to which rifle they came out of… Loss prevention, donchaknow? That poor bastard was soooooo proud of his initiative, after the Air Farce demanded we take the bolts out of the rifles for boarding.

    I think the thing that’s amazed me the most about firearms is how much of the arcana isn’t well-known; I grew up with a rifle in my hands, and knew most of the ins and outs before I even entered high school. The majority of the people I worked with in the Army…? Oh, sweet baby Jesus, save me from the simultaneous profound stupidity and ignorance! I think I spent most of my time as a private biting my tongue whenever we were doing small-arms training, simply because of the sheer wrong of what would be put out by the guys doing the training.

    One thing that would really be good is if someone were to actually lay out precisely how the AR-10 direct-gas action actually works. I swear, that’s got more misinterpretation going on than about any other general subject I can think of. The gas path and purpose behind the way it’s built seems to confuse the crap out of 90% of the people doing the training.

    • Even with Ian’s communication abilities, I’m guessing that explaining headspace is going to be a difficult one.

      Especially explaining why with the same rimless round, eg 7.62×51, different types of gun can have very different headspace tolerances

      And why with different rounds, for example 7.62×51 as a rimless round compared to .303 as a rimmed round, the headspace limit for the rimless round maxes out at around 6 thou of an inch and the rimmed at 10 thou of an inch…

      And explaining in a manner that can be understood by people who suffered a compulsory schooling that was designed to prevent them from ever learning to think, or who have been reading gunzine articles penned by journalism majors, and endlessly follow the same formula;

      “Which is better?”

      (Answer; the product from the company that bought a full page add in this issue 😉 ).

      If there’s a collection of 1900s British shooting papers/ magazines preserved anywhere, I’d love to have a look through. I suspect that the whole headspace fashion / controversy, happened in 1900s Britain.

      Cases that might be used in double rifles tended to have shallow shoulder angles and plenty of body taper, to easily find their way in to the chambers for a fast reload.

      Interestingly the .425 Westley Richards has around a 10 degree shoulder angle for easy loading into double rifles (and there are still some doubles being made in .425 now)

      It also has the length and a rebated rim (it was THE original rebated case) to fit through a standard length Mauser 98 and to load through mauser strippers

      It’s actually loaded a little bit hotter than .416 Rigby and .404 Jeffery, it’s a very capable medium bore big game round designed for things like elephants and tigers.

      Suddenly, and I suspect it follows the marketing hype from Holland and Holland following the introduction of the belted versions of the .375 and .300 H&H cases, there were a rash of steep shouldered, minimum body taper case designs that pre date Ackley and his fellow “improvers” by decades.

      The .416 Rigby, has less body taper than most Ackley “improved” cases and a 45° per side shoulder angle.

      Jeffery produced .333 and 7mm super magnums on the .404 parent case, the same as the American super mags and short mags that arrived about 80 years later. Barnes, COTW damns them, saying something about “there are better and more modern cartridges available”, then goes on to lavishly repeat the marketing hype when he addresses the Remington and Winchester equivalents. Consistency was never Barnes’ strong point.

      The most extreme example of this rush for positive headspacing was explained in a patent by Leslie Taylor, managing director of Westley Richards.

      It was a version of the .318 Westley Richards, with a 90° per side shoulder angle!

      And positive headspace is stated as one of the claims / benefits

      .318 Westley Richards (it’s actually a .33″ diameter bullet) originally had a shoulder angle of around 20°. It’s a similar case to 9.3×62 Mauser: less body taper, steeper shoulder and slightly shorter neck than .30-06, but of broadly similar volume.

      What happened to the headspace craze?
      I’m not exactly sure. I suspect that after four years of mud in the Somme Valley, there weren’t many people who wanted to hear the sound of shooting.

      • “I suspect that after four years of mud in the Somme Valley, there weren’t many people who wanted to hear the sound of shooting.”

        More than likely NO. As I recall, couple years after completing compulsory military service (2 years) I did not want to see anything related to it. This is in contrast with “militaristic” tendencies of some young men in U.S. and Canada. Give them 2 years of compulsory service and you will see the difference!

        Anyway, on subject of head space, and this may relate to what Kirk wrote about, we were strictly warned not to switch bolt assemblies between rifles. It would be unwise, apparently for reason of maintaining correct head space.

        What does that mean? Were our rifles assembled individually from classes of pre-selected parts? I do not know, but my guess is in that sense. In that case not all concerned parts were 100% interchangeable. My later experience on this continent was that most, if not all pieces of equipment were intended to be fully interchangeable.
        On older production rifles you can see that serial number is stamped/ engraved on both receivers and bolts – and for good reason.

        • Very good points, sirs, and I just wanted to add my dollar’s worth. The idea that every gun has interchangeable parts to the point that “any gun can be assembled from randomly selected parts so long as you get a complete gun in the end” is complete and utter bunk. If I were to assemble an AR-15 from random parts on the production line in any munitions factory, chances are more likely than not that I’d have a gun that would NOT work well. Every barrel, every bolt, every receiver, and what-not has tolerance quirks to the thousandth of an inch (or even thousandth of a millimeter), and without fine-tuning, a self-loading weapon hastily assembled from random parts would probably jam on the first trigger pull. It’s just like putting finishing touches on a car or a plane: if done wrong periodically, we’ll wind up having to pay for someone’s funeral (and perhaps a thousand more).

          And yes, Denny, young gun nuts with militaristic tendencies do NOT really want to get drafted. They want the guns, not the drill-sergeant screaming in their ears. They want control, not the status of being controlled. In other words, “My gun, MY RULES.”

          • Thanks for considering my random thoughts…. I like to share my experience, providing there is an interest.

            As it may be obvious by now, I was once (cca 25 years back)involved in initial production of Canadian version of M16. I performed, together with my supervisor, tolerance studies at various groups of parts. We concluded, that parts in order to be interchangeable, they have to be made to extremely tight tolerances. As far as my memory serves me, for instance a distance from nominal diameter .300″ at neck of chamber to locking lugs, when extension is torqued onto barrel was +/- .0015″. That is extraordinary requirement to be done over and over, assembly after assembly. It should be sufficient for illustration what is going on with M16 design. That is not to say the FAL parts (I was involved with them too) were lot more relaxed. I have also seen some parts drawings for AK. Gun parts are demanding for accuracy to make them right, unless you want to custom-fit them. And then there is metallurgy, subject on its own.

          • When we are so far into it…
            to serve in military is almost (with exception of few trips to range) not about guns at all. It is about discipline which demands complete subjugation to higher rank. You are no more than number, just a small cog to fill a specific task.

          • The majority of modern firearms produced are mechanically identical, down to the specifications included in the Technical Data Package for production. The catch is, they’re not identical enough to ensure reliable headspacing or timing, so you have to assemble them with a set of gauges in order to ensure that the things that matter fit correctly. When you are assembling the M16, for example, you need a selection of different bolts in order to test fit the bolts to the barrel extension. With regards to the FAL, you used those little replaceable shoulders the bolt locks up on in the upper receiver. StG57 used a selection of replaceable shoulders on either side of the receiver for the rollers to lock into…

            Where the problems really come in is with regards to the wearing-in of parts; the factory specifications are one thing, but once each individual weapon is out in the field, the vagaries of use/abuse and the recalcitrant nature of mechanical objects takes place–Entropy, in other words. The paired rifles that once might have interchanged parts without problem become separated by wear and the insults of use, such that they are no longer quite the twins they once were.

            Now, what’s interesting is that there are a few weapons out there that don’t require the shooter to worry about such petty matters as headspace–The MG42 series was supposedly one. I was informed as a machinegunner going through familiarization with the Bundeswehr weapons that the MG3 did not require one to match barrels to bolts, because that wasn’t a thing with that weapon. The German Feldwebel telling us that fact suggested that it was because the design of the gun did not depend on headspacing, but I’ve since been told that yes, it did, and the whole “Use any MG3 barrel you happen to find on any MG3 bolt…” thing was due to Germanic diligence in maintenance, and the precision manufacture of the barrels. Supposedly, they actually kept the fleet variables down to the point where they could get away with that, which is actually an extremely impressive technical accomplishment that quite leaves me in awe. Although, the fact that the basic Grossfuss design makes that easier than with other weapons does help…

            FYI, the “timing thing”… My understanding of that has always been that timing is referring to the interplay between the bolt locking up with the barrel, and the hammer or striker being tripped to fire the weapon. Back in the day, with the Browning M1917 or M1919 series of guns, that was adjusted by a little wheel in the receiver that you turned in order to raise or lower the point where the arm that released the striker would hit the release point; same thing on the M2HB. On most other guns, the whole thing is usually performed by fixed mechanical parts fit and interplay, so the gunner need not worry about making it happen–Timing is performed in design and/or at the factory…

            As to the whole interplay of military service and interest in firearms issue? Well, one must remember that ninety-nine and nine-tenths of the people we call “soldiers” are entirely uninterested in firearms and/or that thing we might term “skill-at-arms”, in other than a job-required way. If the particular military force does not demand it, expertise is neither acquired or required, most armed forces being more bureaucrats-with-guns than real soldiers with a vocation for the profession. You can easily be a long-service infantry Non-Commissioned Officer, and quite literally know nothing about small arms past what the system demands of you. Which is why so many bad decisions get made about these things…

            The system really doesn’t demand much, at all. Skill-at-arms is probably one of the rarest things in the military, a fact that is so counter-intuitive that it really doesn’t become apparent to you until you’re the guy who has to try to make up for the legions of ignoramuses on your left and right flanks; you would simply not believe the level of ignorance generally demonstrated by the majority. Not to mention, the magical thinking that all too many display.

            To put it bluntly, I’d say that the actual proportion of military people who are really, truly interested in firearms would have to be somewhere between zero and one percent, and probably closer to zero. In a brigade-sized element, around 3,000 soldiers, I and another “weirdo” were the only ones you could regularly count on being out on the base’s civilian-use range on more than a “zero hunting rifle semi-annually” basis. And, what was bad about that? The other guy who shot regularly, and who was a President’s Hundred shooter? Dude wasn’t even combat arms; it was one of our communications guys who’d spend the entire war sitting in a commo shelter, managing and repairing commo gear…

            The thing which I think would surprise most people with no personal knowledge of military service? The utter lack of interest in weapons or the basics of the profession, as displayed by the majority of the “professionals”.

          • “On most other guns, the whole thing is usually performed by fixed mechanical parts fit and interplay, so the gunner need not worry about making it happen–Timing is performed in design and/or at the factory…”
            Thanks for explanation, this explains why I rarely encounter timing term – naturally all part of machine gun are interconnected, but in most cases I do not see need of naming subset of that interconnections.

      • “Even with Ian’s communication abilities, I’m guessing that explaining headspace is going to be a difficult one.”
        Without going into details, isn’t simply headspace: method of providing correct “deep” of seating cartridge inside chamber?
        But what is that timing thingy?
        Also regarding full-automatic weapons there is yet another confusing issue: ejection vs extraction.

        • See my post above for a partial and probably inadequate explanation of timing…

          Part of this, I suspect, is an issue of language: Firearms terminology ties in with a lot of doctrine, it’s an esoteric field, and the level of misinformation is awe-inspiring. In English alone, you have to worry about the differences in usages of the same terms between the United States, the Commonwealth, world-standard English as she is spoken by everyone else, and the language of idiots who use terms like “clips” interchangeably with “magazines”, without any heed whatsoever as to actual use.

          Generally, though… Extraction is the bit of the weapon cycle where the case is removed from the chamber, and ejection is the part of the cycle where the case is removed from the weapon itself. Your mileage may vary considerably, based on who is doing the talking, and whether or not they actually know whereof they speak…

          It’s unfortunate that this is a field where there is little precision, and that we are using a language whose sole defining characteristic is that it likes to follow other languages down darkened alleyways, knock them over the head, and then rifle their pockets for bits of grammer, spellings, and words that it might find useful. Key defining thing to know about English? There are about a dozen different ways to say anything, they’re all acceptable, and the spellings are artifacts of wherever the speaker happens to have stolen the word… English isn’t a language, it’s a state of mind that borders on schizophrenia.

  3. Going back to basics, time by time, is not a bad idea. If not for any other reason as to reinforce collective perceptions.

    I was one time enthused with idea of retarded blowback semi-automatic rifle breech mechanism and went on to conceptualize it. I thought the mass moving forward was useful complement in resistance against mentioned(retarded) blow-back. I sent the idea to Ian upon which he opined that I’d be facing extended lock-up time, which is true.

    However, let’s face it – for reason of convenience, isn’t it true that many solutions in firearms are a compromise? Well conducted compromise solutions found many adherents in times; one of many being FAMAS rifle. What is your view?

    • “Going back to basics, time by time, is not a bad idea. If not for any other reason as to reinforce collective perceptions.”
      Beyond weapons itself, I would suggest sights overview: i.e. V-notch, diopter sight, peep sight, reflex sight, telescopic sight (what numbers are meaning in for example “8×32 scope”), long-eye relief scopes, night-vision (infra-red), three-dot pistol sight, which sights need voltage source? what is “zeroing” and “wandering zero”? which sights require constant distance from eye and which does not? muzzle-flash vs night-vision and so on.

      • Good idea. Since optical sights are major part of small arms capability increase lately, it makes sense to look at them closely.

        • Sights and what the US Army calls the C3I* complex are probably going to be more important and the larger piece of things going forward than the cartridges and mechanisms.

          *C3I being defined as an acronym denoting “Command, Control, Communications and Intelligence”

  4. Here, for an unlimited time only, you can read MY little idea for closed/open bolt operation: design in a manual bolt latch which at operator’s discretion selects either open or closed bolt firing, independent of semi- or full-auto mode. It shouldn’t add many more parts than FN’s thermostat gimmick.

    • My idea: pump-action style handle under barrel to operate bolt; leave handle in back position and gun fires full-auto open bolt, return handle to forward position and gun fired semi-auto closed bolt. Never remove off hand.
      More radical version: pump-action style handle is connected to a shroud around the entire finned and perforated barrel assembly ending in a finned muzzle brake. Handle in forward position: enclosed shroud and fins act as a suppressor. Handle in back position: fins and muzzle brake exposed to operate normally.

  5. I specifically got a Gevarm because it is open bolt.

    Take the mag out and it is immediately safe.

    There’s no simpler procedure for clearing a gun.

  6. I am not sure whether either system (e.i. open or close bolt firing) is safer. With closed bolt you have to perform 3 steps: load magazine in the weapon, insert round into chamber and pull trigger.
    On open bolt you skip the middle one, meaning shooting happens lot faster. So, which system is safer? I purposely omitted use of manual safety.

    • For clearing a gun with a magazine, open bolt is definitely simpler.

      Just pull out the magazine.

      There’s no chasing around after an ejected round,

      and unless there’s been a hang fire or a failure to fire, there’s no chance of leaving a loaded round in the chamber.

      • Yes, but you better make sure your fire control is on safe all the time. Unintentional/ accidental sear release means you have one out of pipe. Most of accidents historically happened during unloading.

        • In other words, a pratfall with an unsafe open-bolt gun and a loaded magazine results in “Oh no, I just shot Alfred in the knee!”

          • Luckily, there are not too many rifles operating with open bolt concept. To me just the pure essence of it being that the gun is accidentally jarred while magazine is still in place is scary.

            My own experience with vz.58 while unloading it during sentry service was uneventful. It was done away from guardhouse at designated station, pointing in safe direction. When soldier was entering guardhouse, his mag was in pouch and weapon was on safe.

    • “not sure whether either system (e.i. open or close bolt firing) is safer”
      Depending which safety do you mean.

      Regarding being drop-safe open-bolt design have poor record there, I read about cases of striking PPSh against ground/floor (barrel pointing up) which caused unwanted shots. IIRC there were also some problems with unwanted shots in STEN machine carbine. But in reality this have more to do with design of given gun that fact of being open/closed bolt – take for example MEKANIKA URU which we already discussed earlier:
      which is both open-bolt design and is drop-safe (no shots by striking hard surface)

      Regarding cook-offs, it is true that open-bolt design prevents such accidents, but on the other hand there were many machine guns using closed-bolt design and being enough resistant for that mishaps. To give examples:
      samopal vz. 61 (7,65 mm Browning alias .32 Auto)
      RPK (7,62×39 mm pattern 1943)
      HK21E (7,62×51 mm NATO)
      Vickers (·303 British)

      • “Depending which safety do you mean.”

        You are correct. I was a tad ambiguous. Taking on first generation SMGs is good example how dangerous to user the open-breech firing guns can be, in almost all circumstances. STEN was a notorious case in that regard.

        • It gets worse when people do Hollywood reloads for long arms. I refer to using the off hand for magazine changes and bolt charging while the trigger finger is still on the trigger. The result is a cool-looking sequence but also a possibility of shooting your friends in the kidneys by mistake!

  7. good video! i am glad you decided to do these. it would be very beneficial to have all this information on one site and from such a professional host.

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