The Firearms BlogSince 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:
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.
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. 
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:
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:
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:
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.
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.
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:
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:
Comparing the bolts side by side shows a number of key differences as well:
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:
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).
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!
– See more at: http://www.thefirearmblog.com/blog/2013/06/20/open-bolt-explained-tale-uzis/#sthash.d5O3TVDG.dpuf