“Open Bolt” Explained: A Tale of Two Uzis

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Since I got into machine guns one of the most unusual things I have noticed is that an astounding amount of people who shoot most commonly available submachine guns will get ready, anchor their feet into the ground, cock the bolt to the rear, and then look at me confused and say “it won’t cock” or “the bolt won’t close”. I used to simply say “oh, it fires from an open bolt,” but that typically led to more confusion, even among people you would assume would be familiar with that type of action (including military folk and police officers). Hell, I have been to competitions where the range officer was insistent upon me walking around the range with the gun’s bolt open in the ready to fire position, even though I explained how the bolt closed with no mag in it is correct and that to render the gun completely safe I would have to remove the magazine and walk the bolt forward. Nowadays instead of explaining how an open bolt gun works I just say “it’s fine, just pull the trigger” and people will shrug it off and dump a magazine.

The reason so many American shooters are unfamiliar with the way open bolt guns work is because the ATF made a ruling in 1982 that semi-automatic open bolt weapons are “readily convertible to fully automatic fire”, therefore such weapons manufactured after the date of this ruling got classed and controlled as fully automatic weapons (weapons manufactured prior to the ruling are grandfathered and are still considered semi-automatic but bring big money as curios). In the ATF’s defense, Thugs could take an over the counter mac 10 or tec-9 and easily (and I do mean easily) convert it to fire full auto, so they got banned. The Macs, Uzis, and Thompsons you see today in stores fire from a modified closed bolt system that is so dissimilar from the weapons original design that they share little in common aside from their external appearance. And I will be damned if that isn’t a fact; Unless you are an Uzi guy it would be hard for you to tell which of these is a selective-fire open bolt gun and which isn’t:

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Both of these firearms share many cosmetic and superficial parts in common, they fire the same cartridge, they utilize the same magazines, and they were both made by the same manufacturer! One of these guns is a Group/Vector SMG (one of about 3,500 on the registry from what I understand) and the other is a Group/Vector SBR (not a factory SBR but a gun that I bought as a carbine and put on a Form 1 and had the barrel turned down).

Open bolt is by no means a fancy operating system, in fact there are less moving parts and it is less sophisticated than a hammer or striker fired system, but there are some distinct advantages and disadvantages:

Advantages-
Fewer moving parts
The firing pin is usually part of the bolt
In automatic weapons an open bolt helps eliminate cook-off
Open bolt designs typically operate much cooler

Disadvantages-
The weapon is more prone to fire when dropped.
Subject to picking up dirt.
Open-bolt machine guns can not be synchronized to fire through the arc of a propeller.
Accuracy can suffer in an open-bolt design, but this is less of a concern in automatic weapons.              The large mass moving forward kicks the gun forward a bit on single shots.

 

The boring and lexical definition of the open bolt operating method is: “A semi or full automatic firearm is said to fire from an open bolt if, when ready to fire, the bolt and working parts are held to the rear. When the trigger is pulled the bolt goes forward, feeding a round from the magazine into the chamber and firing it. Like any other self-loading design without an external power supply, the action is cycled by the energy of the shot; this sends the bolt back to the rear, ejecting the empty cartridge case and preparing for the next shot.”

So yeah, this is one way of explaining it, but as they say, a picture is worth 1,000 words, so I tried my best to demonstrate the above definition by comparing and contrasting a pair of Uzis.

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Here we a pair of delightful 9mm firearms that have brought this writer much joy and enjoy an equal amount of range time (despite one being a semi, the closed bolt gun does allow for great accuracy and is a joy to shoot).

First up is the selective-fire gun. IMG_0947

In this position (if a loaded magazine were inserted and the safety was off) the gun would be ready to fire. The Uzi’s heavy bolt slams forward with as much if not more force than a 115 grain 9mm round recoils the gun rearwards. In other words, it is very easy to keep an Uzi on target.

The Uzi SMG is perhaps the easiest gun to clean I own (that isn’t a bolt gun or single shot) because this is all you need to do to field strip it:

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So you have a bolt (and recoil spring), the receiver group, and the top cover. You can easily remove the barrel and grip but I generally don’t unless I just shoot it an absurd amount. To clean this gun I usually just hit the barrel with a Hoppes soaked brass brush and then a bore snake with some CLP on it, then I wipe the bolt down with a rag and some rem-oil. This has worked well so far, as the gun’s only malfunction has been when I had a genuine dud (primer failed to ignite).

Anyways, as stated above the firing pin is a part of the bolt:

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So when the trigger is pulled, the bolt slams forward, strips a round from the magazine, and keeps on chugging. It really is mind-blowingly simple and I have taught people how to field strip, load, and shoot this gun in under ten minutes!

So anyways, onto the semi-auto version:

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A few things should jump out when you see a semi auto Uzi copy. First of all you will notice the bolt carrier has a slot milled into it, and you will see why in just a moment. They also lack a ratcheting top cover (as semi guns don’t need that extra safety feature) and the selector will not have a third setting for rock-and-roll. Now this gun, being of a closed bolt design could fire from this position if a round was in the chamber and the selector was not on safe.

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Jumping into the guts of the semi we see that there are two springs, as opposed to one on a true SMG. One is to drive the bolt forward and strip a fresh round from the magazine, while the other is for the striker mechanism. When this firearm is cocked, the striker is locked rearward until the trigger is depressed, and then the striker hits the primer and the entire assembly flys to the rear and reloads another round into the chamber which remains there until the shooter pulls the trigger again.

IMG_0970

Here you can see the semi auto Uzi field stripped and you can clearly see the striker mechanism (which fits onto the bottom of the bolt in a section that has been milled out to accommodate it).

I mentioned earlier that the slotted bolt is a giveaway that the gun is probably a semi (there are some slotted bolts out there registered as machine guns however) and here is why:

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Semi guns have this bar, known as a blocking bar welded into them to prevent the installation of a full auto bolt. Here you can see the SMG lacks this bar:

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Comparing the bolts side by side shows a number of key differences as well:

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Notice the differences in how the firing pin is setup on the respective bolts, and the slot milled into the semi auto bolt to accommodate the striker’s arm. The sear engagement surfaces are also in different locations, a part of the bolt face on the semi is milled off, and the semi bolt has additional milling inside to clear the barrel restrictor ring:

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This is the semi automatic’s barrel and feed ramp. It is a milled piece of metal that helps prevent the installation of a short SMG barrel (that lacks a turned down section of barrel at the rear).

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Here is the SMG barrel. The feed ramp is actually just a raised section of receiver. Ironically many guys have the factory ramp cut out and have a milled ramp put in (minus the restrictor ring) because they will feed a wider variety of ammunition (including hollow points) better.

So that about covers the most significant differences between an open and closed bolt Uzi. While more exist, these are the most significant. It is also worth nothing that some gunsmiths have made selective fire closed bolt Uzis, but these are quite uncommon and I have never seen one in person. Regardless, I hope you enjoyed this article, and that if you get your hands on an open bolt firearm of some kind you can confidently look the owner in the eye with that “I know what I am doing” smirk of confidence!




Alex C.

Alex is a Senior Writer for The Firearm Blog who was born and raised in Texas with years of experience in hunting, shooting competitions, and general collecting. A degree in History from Baylor University has contributed to his love of both early and modern firearms technology, but Alex is most fond of machine guns and other NFA toys. Alex also owns a firearm manufacturing business licensed to produce title I and II weapons.
You can reach Alex at [email protected].


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  • Rob Reed

    The ironic thing is that open bolt subguns are so simple to manufacture that they would be the easiest for a home gunsmith to manufacture in the event of a more or less “total” gun ban. A closed-bolt, semi-auto is actually harder to build from scratch.

  • Charlie F.

    Hey Alex, great comparison. An open bolt SMG has another advantage. I was a member of the NYPD SCUBA Team. We used to train with S&W M76. The open bolt was great when emerging from the water. We would use them while inspecting various areas on ships or when searching under piers. It was very finicky if handled wrong. It really did require a lot of training.

    • Alex C.

      That is really cool Charlie! I have always wanted a S&W 76 or an MK 760 but other stuff always pops up first. I did not think about the advantage of an open bolt when emerging from water, but it makes great sense. Thank you for commenting, I love it when readers tell me about their experiences!

    • Anon. E Maus

      I believe it was either MACVSOG or the Navy Seals that wanted the Swedish m/45 smg in Vietnam for this exact purpose, but we put an embargo on you guys due to political reasons, so Smith & Wesson moved in to produce a similar product, the M76, but alas, their interest had waned, and S&W was too late.

  • Cornelius Carroll

    Huge fan of the original UZI design. Incredibly simple to make and very reliable.

    • http://www.thefirearmblog.com/ Phil W “Senior Writer TFB”

      Very reliable, accurate and very easy to fire on full auto and still fire a very decent group. I had the opportunity to own one for some time in the mid 1980’s. Mine had a wooden stock which made it that much more controllable.

  • Tomas J. A. L.

    Informative article Alex.

    • Alex C.

      Thank you Thomas, I am glad you enjoyed reading it!

  • PR

    “…Can not be synchronized to fire through the arc of a propeller”. I forgot it was 1918.

    • http://www.thefirearmblog.com/ Phil W “Senior Writer TFB”

      An interesting historic fact is why that was included.

    • Ripley

      Synchronizing fire through the blades of a jet engine is an even bigger engineering challenge today.

    • Alex C.

      It was just an interesting factoid I included that I thought was kind of neat.

    • PR

      Sorry, just a little dry humor ;)

    • hailexiao

      It’s something to consider when you want to gun up your Cessna Skycatcher.

  • Ben 10

    the 1934 and 1986 NFA acts should be repealed and abolished. that way civilians will be allowed to own SMGs again.

    • http://www.thefirearmblog.com/ Phil W “Senior Writer TFB”

      You can now it’s just a long process and a hassle.

      • Drew

        well then, it should be made swhorter!

        • http://www.thefirearmblog.com/ Phil W “Senior Writer TFB”

          I know you’re preaching to the choir. It’s way more time consuming than it should be but that’s what we have to deal with. This part of owning guns and suppressors won’t ever change to make it easier.

          • HSR47

            I’m not convinced of this.

            Over the last few years we’ve seen a continuous improvement in the legal status of NFA items in the various states, combined with an expansion in the number of states in which it is legal to hunt with firearm mufflers.

            This, combined with an expanded interest in NFA-regulated devices (thanks to inflation, $200 just isn’t as big of a deal as it once was, and trusts/corps help get around some of the arbitrary issues about multiple parties having independent physical access to NFA items) has lead to a massive increase in the number of NFA items being created and transferred on a yearly basis.

            As it stands today, a growing percentage of the population owns firearms (and among those that don’t, the percentage that is staunchly anti-gun is shrinking), and a growing percentage of gun owners owns one or more NFA-regulated devices.

            Those who already own NFA-regulated devices, and many more who do not, know that the NFA process is, as a whole, arbitrary and capricious.

            Assuming that these trends continue, it is likely that they will result in legislative, judicial*, and/or executive**.

            *As the rate of NFA ownership increases, it will arguably reach a point at which the courts will be unable to deny that suppressors and short-barreled rifles/shotguns are “in common use.”

            **We’re already seeing action within the BATFE to increase staffing in order to handle the massive growth in the NFA marketplace since the end of the Bush Administration.

      • BryanS

        Not with the artificial market forces that the 86 addition created.

      • Jeremy Star

        Not to mention ridiculously expensive.

    • Alex C.

      We can own them now. It has never been illegal to own machine gun on a federal level in the United States. It is however difficult and expensive. If you want the NFA repealed and the ’86 Hughes Amendment repealed then write your congressmen.

    • Vermin.308Winchester

      Well yeah but how do we do that they just tried to push complete insanity upon us we try to quietly murder I mean retire any full auto legislation and they will completely lose their shit I mean we can’t even deregulate suppressors on a federal level. Best bet is something like that strange thing they tried to pull off in Montana. Want one cheap In the mean time? The your going to have to hit the home depo or convert something gas operated

  • Blake

    nice article, & great photography.

  • Legersois

    Thanks for this article. Really informative and well explained =)
    UZI is on my wishlist !

  • noob

    I’m curious about the stock on both the uzi variants, does it unfold in a Z shape? And how does it lock open? (I’m guessing there is some kind of dimple in the pressing around the hinges that lock together stiffly when opened)

    As the open bolt lurches forward on the first shot, does the stock noticeably move from your shoulder?

    • Joe

      Yes, it folds in a Z shape and locks open with a push button release. The “pros” know how to “slap” it open and deploy it quickly.

      • Vermin.308Winchester

        also quite a few holster company’s made rigs that did that for you

    • Vermin.308Winchester

      You should watch some YouTube videos on this stuff it’s mostly all their and you can learn almost all the manipulation details for everything from the m2 browning to the webley foseberry. Anyway if you want to go more in depth try Forgotten Weapons if not LifeSizepotatoe is your man he recently taught me a few things about the Korth company’s unique cylinder realease which isn’t something Ian would cover

      • noob

        Thanks for introducing me to those youtube channels and the folks behind them. These are great!

  • Cuban Pete

    The blueprints for the STEN is available online (I downloaded my own copy) …. just in case you need it in the future….if you know what I mean!

  • Rick

    I take it that the standard bolt is a controlled item as well? Otherwise, it seems comically simple to repair

  • http://www.thenewrifleman.com/ Lothaen

    I too dislike purchasing firearms that have been re-engineered over their military counterparts. With the AR15 it is largely unchanged sans the auto sear and assc. parts. With something like this there is a big change which is not as attractive in some platforms.

    With the UZI at least there are many spare parts as to make the point moot i guess.

    • Esh325

      I don’t know if I can attribute this to the closed bolt firing, but a Polish PPS43 pistol I got was very unreliable compared to the original open bolt one.

      • HSR47

        It’s likely a combination of being imported as a parts kit (sans receiver) and then carelessly modified to fire from the closed bolt.

        This type of conversion/manufacturing is typically done with the goal of having the end product be as inexpensive as possible, which tends to result in guns that are often fairly unreliable.

        Such conversion/manufacturing can certainly be done right, but the end result is a relatively expensive firearm, which has an extremely limited market appeal.

        In other words, there’s a reason why a CAI FAL is so much cheaper than a DSA FAL.

  • Bukowski

    Another important item I try to teach people who are unfamiliar with open bolt firearms is to control the bolt first. If you have a FTF don’t pull the mag, control the bolt first. I find that people who don’t shoot open bolt guns often tend to pull the mag before locking the bolt,
    which can lead to accidentally firing a round if the gun is halfway through its firing cycle.

  • avconsumer2

    Great read. Was unfamiliar with open bolts until my recent 240b experience. Good times.
    Also, Uzi… what an amazing design – all variations.

  • Joe

    The only thing missing is a picture of the underside of the top covers. I have a semi, but I’d like to see the difference with the ratcheting mechanism.

    • Alex C.

      The top covers are the same, but on a semi a little arm under the actual knob is removed so they look identical externally.

  • tincankilla

    this kind of post is what makes this blog so awesome. can’t wait until these kind of designs start benefiting from 3D printing…

  • Esh325

    I think closed bolt firing has won the day though with regards to SMG’s,PDW’s, and assault rifles. Almost all modern SMG’s use a closed bolt. Open bolt assault rifles have been tested before, but they weren’t satisfactory. Open bolt firing seems only regulated to light machine guns and heavy machine guns today.

    • Vermin.308Winchester

      Fabriqe nationals heat adaptive assault rifle concept has enourmous however poorly named as it may be.though it got little attention due to its attempt to be a designated marksmens rifle and a SAW at the same time this too has merit however only for special forces and other high skill users as you have to be able to make it work in both open and closed bolt modes. Is their such a thing as a manually operated version of the same concept I mean why automate it

    • dp

      This is true for plain and dull solutions. There is however other transient region of design and this is “advanced priming”. It has obvious advantage when using momentum forward to quell momentum of recoil. Its kind of silly not to use it, isn’t it? If somebody masters this reliably with sufficient functional margin (with allowance for wear and dirt + tolerances), it could be applicable for current rifle ammunition. Here we have another possible development direction for assault rifles. Why to keep locking chamber (and getting kick) with fancy and costly mechanism if not necessary?

      • DiverEngrSL17K

        Are you referring specifically to “advanced primer ignition” rather than “advanced priming”?

        • dp

          In my understanding of terms, this is identical. Term “advanced priming” sounds like short for the former. Ignition time is so short that mechanical action of striking can be considered inseparable from flame initiation.

      • Esh325

        If they could have an open bolt that’s just as protected as closed bolt, it would certainly be advantegeous.

        • dp

          Certainly. Also as a person of knowledge you know that designers typically leave for themselves a ‘way out’. This part can ascertain that full safety is in place plus balance out the system. This add-on device could have form of retarded blowback. I had been long time involved with design of this type; of course nobody will build it for me. If (retarded) military is not interested, nobody else is.

          • Esh325

            I think the reason why blowback has gone the way of the dodo bird is because they don’t work very well with cartridges of relatively high pressure cartridges with high bolt thrust.

    • DiverEngrSL17K

      Some machine guns fire from the closed bolt, eg., Browning’s venerable M2HB .50-cal. HMG and its smaller brethren, the M1919 .30-cal. MMG.

      • Esh325

        Didn’t say there weren’t closed bolt machine guns.

  • Jeremy Star

    This is another thing right up there with pistols in TV and movie mistakes. Shot one: Bad guy racks the bolt and the bolt ends in a closed position. Shot two: Bad guy with an open bolt, starts shooting. Shot three: Bolt closed again. But bad guy still threatening to shoot more.

    With pistols it’s the very noticeable fire two shots and the slide locks back, change camera position and the slide is once again forward. Some editors are just not great at fixing the on-set safety rules in post.

    • noob

      of course, if in shot three bad guy is called to point a gun at the good guy the fight director has to be very careful to avoid pulling another “The Crow”

      my uninformed mind thinks of a couple of options

      1) replace blank firing gun with rubber replica. The Matrix loved this one. A rubber replica uzi might be depicted with the bolt closed, although that’s up to the replica maker and you’re stuck with the way the replica looks.

      2) make the gun safe and point it. once the footage is taken that way, you’d need CGI to correct it. the producers would probably say “I’m not paying to punch in a CGI gun over the perfectly good ‘real’ uzi. It’ll just make the guys at imfdb have something to talk about. this scene is all about emotion anyway”

  • Capybara

    Great article. But old news to anyone who frequents http://www.uzitalk.com The Uzi is now basically considered an antique in the world of NFA items but they are reliable, relatively inexpensive compared to an MP5 and have an amazing history.

    If I lived in a free state, I would own an FA Uzi but the state I live in restricts them to LEOs and movie studios.

    • Daniel Alan

      ill take no right to bear arms for 500 please alex

  • Isaac

    Great article, I have a better understanding of my Vector Arms semi-auto. Great gun for $600. :-D

  • DiverEngrSL17K

    An excellent, very well-written and nicely-illustrated article. Although I am entirely familiar with the differences between the open-bolt and closed-bolt actions, I nevertheless truly enjoyed reading your post on the Uzi SMG’s. Congratulations on a concise and informative analysis and thank you for adding to the firearms knowledge base!

  • milo

    very intresting, good job writing it!

  • John

    Interesting open bolt story:

    I own a Cobray Terminator. They’re pretty rare with, based off of my research, less than 100 produced (there is a specific number, 14 I believe, floating around the Internet but no actual proof that that’s how many are out there) in the mid-1980s. Basically they’re spare AA-12 barrels from a pulled government contract that Cobray turned into these Sten-looking single shot shotguns.

    The Terminator used a rather unique to shotguns method of firing. You move the “bolt” (and by “bolt” I mean the entire barrel) forward, compressing a spring until it locked in place, the bolt and chamber open. You then load a round directly into the chamber and pull the trigger, releasing the spring and sending the barrel slamming rearward onto a fixed firing pin. The ATF determined this was open bolt and thus easy to convert to full-auto, despite this firearm being single shot only. It would be easier to build a fully automatic shotgun from scratch than to convert the Terminator into one, with the way it’s designed. The ATF follows the letter of the law, if not the spirit. So, Cobray was forced to halt production, with only a very few Terminators entering the market.

    It’s an ugly thing, but it has its charm. It is NOT fun to shoot, though, being a 7.5-pound sheet metal shotgun with a metal telescoping stock taken off of one of Cobray’s many MAC-10 clones and an action that only adds to recoil.

    • Cymond

      I’ve read about the Terminator before, but one question lingers: why? Why would someone choose an awkward, single-shot shotgun? Were they really affordable at the time? They look neat, but they seem like one of the least practical designs possible.

      • Vermin.308Winchester

        It’s the same logic as the replica liberators romance in revolutionary tools is understandable for the surfs of the Clinton totalitarian crusade this being a slam fire shotgun based on the pipe guns of various s of many revolutions notably those of several soveit splinter states again it resonated emotionally during a time of great loss failure and concession

      • John

        Based off of my research, they were available for sub-$100 in the late 1980s (not anymore thanks to the very limited supply – I paid $450 for mine which is an all right deal in 2013), and Cobray made them because they already had 12ga barrels that the government had paid them to make, so costs were very low for the Terminator.

        Cobray tended to make scary-looking guns almost exclusively, so the design of the Terminator fits their MO.

        I wasn’t alive in the 1980s at all so I can’t provide any first-hand insight into the gun culture during that period, just what I can figure out based off of my research and knowledge of the Cobray Company.

      • Anon. E Maus

        I figure it was to cut losses for what would otherwise had been wasted shotgun barrels.
        I mean, they could have made an actual nice shotgun, but I guess they chose to just take the easiest route possible.

  • Mazryonh

    Why exactly do Open Bolt firearms run cooler and generally less accurate in semiautomatic fire than Closed Bolt firearms again? Plenty of webpages talking about them will say this, but do not explain why.

    • Cymond

      When the gun is not firing, the action is held open, allowing for more ventilation and more cooling. The accuracy problem (especially when firing single shots) comes from the bolt slamming forward before each shot. When the bolt slams forward, the gun jumps in your hands.

  • perlhaqr

    So, the firing pin is totally fixed as part of the bolt? What prevents it from firing out of battery? (I.e.: before the bolt has finished closing.)

  • tyrone

    > has awesome uzis
    CAN I BE YOU NOW?

  • Lm Simmons

    The mini UZI is a closed bolt FA arm as is the Micro UZI. Rate of fire is higher with the Mini and even higher with the Micro! And it is possible to convert a semi bolt to use in a FA gun, with a little welding on the bolt face. Not as reliable or long term as the original open bolt model, but possible…. I bought mine (FA) based a friends long term usage of a dealer sample he has. Uncounted thousands of rounds down range! Long term investment I can enjoy, with, thanks to that idiocy in ’86, no appreciable loss in value. Win-win!!

  • Mr Silly

    The myth of the Uziel Gal continues. Why no mention of the M23/25 or ZK 476 Gal openly admitted copying?

  • Franco

    Excellent explanation! Thanks for sharing
    Greetings from Argentina

  • Anon. E Maus

    >Thugs could take an over the counter mac 10 or tec-9 and easily (and I do
    mean easily) convert it to fire full auto – See more at:

    I think this has been exaggerated to a great deal, while open bolt MAC-10 pistols did sell in some quantity, and some were illegally converted, the open bolt Tec-9’s saw very limited sales before the ATF made them change their design to closed bolt, the initial KG-9 is rather rare, with only a couple of thousand having been made, the KG-99 (closed bolt), is somewhat unusual as well. I’m sure illegal conversions did happen, but not nearly as much as some would suggest.