The bolt in a firearm is the movable part which contains the firing pin, and often other items as well (such as the extractor and/or ejector, which do just about what their names imply). A bolt-action rifle is so called because the shooter operates the bolt by hand, to remove a fired cartridge and insert a new one (either by hand or from a magazine), but a lot of other types of firearms have them. The functional part of many semi-automatic rifles, and a few handguns, can be considered bolts—for instance, the reciprocating part of a Nambu or Ruger automatic pistol, which is contained within the body of the firearm instead of being part of a slide assembly.

"Open-bolt" and "closed-bolt" refer to the ways in which two different types of operating system handle the way the bolt works. Of the two, closed-bolt systems are by far the more common, so we'll start with that. In a closed-bolt system, the firearm's normal state when ready to fire is, as the name implies, for the bolt to be closed (and, in locked-breech systems, locked). The bolt face is seated against the chamber with the ready-to-fire round completely enclosed, and some form of ignition system, usually a hammer or striker, is cocked and ready. When commanded to do so by the fire control system—customarily the shooter's finger on the trigger—the ignition system will send a firing pin forward to strike the chambered cartridge's primer, the cartridge goes off, and the operating system does whatever it's going to do (either automatically or by user intervention, as designed). The end result, if all goes according to plan, is a return to the same ready-to-fire state with the next cartridge along, if there is one.

This is how all manually-operated repeating firearms I know of, and the majority of modern automatic ones, work. Anything that needs a locked breech, which is to say most handguns chambering 9mm Parabellum and up or any small arm using any but the weakest rifle cartridge, will have to work this way, for the obvious reason that the action can't be locked if it isn't closed.

In an open-bolt system, the ready-to-fire state is one in which the chamber is empty, there's a round standing by at the top of the magazine, and the bolt is locked open. Pulling the trigger releases the bolt to go forward under the pressure of the recoil spring; it scoops up the ready round as it goes forward, chambers it, and then immediately fires it upon closing, usually because the firing pin in an open-bolt firearm is fixed and cannot help but strike the primer when the action fully closes. The action is then immediately blown open again by the cartridge pressure; the bolt travels backward, extracting and ejecting the spent case as it goes. (In most blowback systems, it was coming out anyway.) When it gets all the way back, it either locks in place or turns around and heads forward again, to pick up and fire the next available round.

As the description above may imply, this system particularly lends itself to full-auto blowback actions, though it's possible to create a sort of accidentally-pseudo-open-bolt locked-breech system if you have a stuck firing pin on, e.g., an SKS carbine. That replicates the fixed firing pin of an open-bolt full-auto system, with the added fun benefit that you can't cause the bolt to lock to the rear, which means you're firing full auto until you run out of ammo whether you like it or not. As I mentioned in the SKS entry, this is called a slam fire (for obvious reasons) and is generally not a prized outcome.

(Although contrary to some people's beliefs, it won't get you into trouble with the ATF. A semiautomatic firearm suffering a mechanical malfunction is not a Class III weapon. If it got that way because of poor maintenance, the people running the range it happened at would be justified in throwing you out, but they probably wouldn't call the cops. Now, if you jammed the firing pin of an SKS on purpose... well, you'd be an idiot, because you'd have made an inherently uncontrollable machine gun, on top of whatever legal problems you may incur.)

Anyway, you don't see open-bolt systems all that often any more, but they enjoyed a vogue in the 1930s through the early 1960s as the operating system of choice for many submachine guns. Name a World War II submachine gun, and it probably fired from an open bolt—Germany's MP38/40, the British Lanchester and Sten, the American M3 "Grease Gun", and the Soviet PPSh-41 and PPS-42/43 all were widely used open-bolt subguns. (Notable exceptions are the American Thompson and Reising SMGs, which both fired from a closed bolt—although except for their use of a pistol cartridge, they were both practically carbines.) It's an easy system to make, requiring fewer moving parts than a closed-bolt system; the fire control group, in particular, can be laughably simple, since all the trigger has to do is let the bolt come forward or not. You don't need a separate hammer or striker spring, or the mechanisms to operate them; inertia and mainspring pressure do everything.

(As such, the ATF does not much care for open-bolt semiautos, precisely because they are so trivially easy to convert to full auto.)

There are disadvantages, though, the most notable of which is that open-bolt guns tend to be hilariously inaccurate. Partly that's because they also tend to be short-barrelled SMGs, which are not known as surgeon's tools no matter what their operating system, but beyond that, they're just awkward to shoot with any marksmanly technique. There's a perceptible delay between pulling the trigger and getting a noise; the bolt has to be released, accelerate, and come alllll the way forward before anything happens. It doesn't take half an hour, true, but still. I'm told it's a bit like firing a flintlock, in that you have just enough time to think about it—and foul up your shot.

That same fact also means that the weapon's center of mass is shifting before and after the shot, rather than just after as in a closed-bolt system. Sometimes, since blowback SMGs tend to have big heavy bolts to provide the required inertia, it shifts by quite a lot. The Grease Gun, being chambered in .45 ACP, was particularly notorious for this. So was the Soviet PPSh-41, which was in 7.62mm Tokarev and had an insane rate of fire as well.

They're also reportedly a bit disconcerting to shoot, in that they don't stop firing the instant you let up on the trigger; because of the way the system works, if the bolt was on its way forward when you released, you're getting one more round before it stops. This was not seen as a terribly big deal back in the day, because these were not weapons for precision shootsmanship anyway, but in today's infantry doctrines immediate no-more-bang is at least as important as immediate bang, if not more so.

A further common problem with open-bolt submachine guns, rediscovered again and again during World War II, is their nasty habit of going off if dropped. Without some form of positive locking safety (which most of the common ones of the WWII era don't have), the bolt is just hooked there, with all the spring tension in the system wanting to drive it forward. If the gun happens to be jarred hard enough to cause the trigger mechanism to disengage, it will immediately slam shut and fire the first available round. If you're really unlucky, the drop breaks the trigger system and you have a loose bullet hose flailing around on the ground until it runs out of ammunition. Not a good scene.

Nevertheless, the open-bolt principle was very popular with SMG designers in the mid-20th century. Nowadays there aren't many open-bolt guns still in service, and as I previously noted, civilian versions are not looked on with much enthusiasm by firearms regulators even in the US, so except in some very specialized and/or historical contexts they're pretty rare. It would be very surprising if any item you found in your average American gun shop these days fired from anything other than a closed bolt, assuming it had a bolt at all.

--G.
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Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
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