View Full Version : Firing from open bolt vs. closed bolt?

May 12, 2007, 03:26 PM
I've tried to school myself on what that means, but can't visualize the difference between firing from an open bolt versus firing from a closed bolt. What's the difference, and does anyone have a .gif-type animation of what's going on? Does an open-bolt only apply to FA?

May 12, 2007, 04:01 PM
Its quite simple. With the open bolt moves when firing and the firing pin is not a seperate component. You pull back on the bolt to lock it in place and the mechanism will be open to the air. When you fire the gun the whole bolt moves forward. If its a semi automatic or automatic weapon than the bolt will be pushed back open after firing.

With a closed bolt system it has a moveable fireing pin, so when loaded and made ready the bolt is closed all the way and the firing pin hits the primer and goes back to a closed position after firing.

Open bolts are almost always used on automatic weapons and its not too difficult to convert an open bolt system that doesn't to FA so the BATFE class all open bolts as a FA reciever. Open bolts also deal with cooling better.

Closed bolts are more reliable as you don't have the whole bolt slamming into the reciever to fire the round. Most firearms you buy will be closed bolt.

I am sure someone can explain it simplier than that though with pictures on the forum.

May 12, 2007, 04:14 PM
With the open bolt moves when firing and the firing pin is not a seperate component

I don't mean to confuse things but thats not entirely true - rifle caliber open bolt weapons do not have a fixed firing pin. In any open bolt rifle caliber weapon, the firing pin has to move and there needs to be some mechanisim to release the firing pin after the bolt goes into battery, and with enough force to set off the primer. Pistol caliber open bolt guns are slightly different in that because the pressures are lower, and there is usually no shoulder on the case, there is less chance for an out of battery detonation that can seriously injure the shooter (I've seen it happen though). With a shouldered case in a rifle caliber, the chances for an out of battery detonation are much higher and the risks to the shooter are much higher.

May 12, 2007, 06:06 PM
If your not familiar with them, open bolt guns can be a little hairy, safety wise. They have a manual of arms that is different than most are accustomed to.

When the open bolt gun is ready to fire, it will appear "safe", to those who are familiar with most semi auto guns. You also have to pay attention as to how you handle them. If your hand slips while cocking, the gun can discharge, and if your finger is on the trigger, the gun will continue to do so until released, or the gun is empty. Its always important when shooting them (or any FA for that matter) to take your finger off the trigger when the gun stops. If you should have a hang fire, (I've seen this happen using old ammo) and it goes off after the fact, the gun can/will start running again.

Also, if dropped, especially butt first, with the bolt down (safe, more or less) on a loaded mag and the safety isnt locked, most of this type gun can discharge. All the bolt needs to do is come back far enough to strip a round, and it can fire when it goes home.

You have to remember to pull the mag BEFORE you pull the trigger to lower the bolt when making the gun safe. Sounds silly, but I've seen it done more than once.

I wouldnt call them "less reliable" than a closed bolt gun, if anything, most are usually a lot simpler and have less working parts. Some are downright crude, but very functional.

The other complaint you often hear is they are less accurate. For the most part, they are not as precise as the closed bolt guns when fired singly, but they are not necessarily inaccurate.

May 12, 2007, 09:04 PM
Open bolts also deal with cooling better.

Why is that the case? Open or closed, the bolt still moves back to chamber another round. In fully auto, I can't imagine why cooling is better, the only difference is a fixed or separate firing pin.

Only way this is true is if you fire one round, bolt stays open in open bolt design, bolt open and closes for closed bolt. Then technically cooling is better after. But if you are only firing one round, it doesn't matter.

May 12, 2007, 09:10 PM
But if you are only firing one round, it doesn't matter.The only open bolt weapons I'm aware of are fully automatic.

The bolt staying open at the end of a burst allows air to circulate better and the parts cool more rapidly.

May 12, 2007, 09:12 PM
The only open bolt weapons I'm aware of are fully automatic.

BATF stepped in mid-1982 and halted the manufacture of open bolt semi's because they were easily convertible to full-auto.

Wasn't always the case.

The bolt staying open at the end of a burst allows air to circulate better and the parts cool more rapidly.

True. I was just questioning the cooling effect during firing. No difference between closed or open bolt designs.

May 12, 2007, 10:08 PM
Open bolts deal with cooling better? if that's true I would like an explaination on how.

May 12, 2007, 10:15 PM

May 12, 2007, 10:24 PM
Open bolts deal with cooling better? if that's true I would like an explaination on how.

Kind of like how opening your windows allows a breeze into the room.

May 12, 2007, 10:34 PM
It's a bit misleading because the gun does NOT actually "FIRE from an open bolt". It closes the bolt FIRST, then fires, with a single pull of the trigger. If it fired from an open bolt, the cartridge would explode in the open receiver. But it does sort of "fire" if you define "fire" to mean, "start the firing process and then fire, with a single pull of the trigger."

May 12, 2007, 10:49 PM
So, it's kind of like a controlled slam fire? And I assume the bolt has an extractor? What's the benefit?

May 12, 2007, 10:57 PM
FWIW, I do have an open bolt semi-auto, manufactured before the 1982 proscription. A description of firing from an open bolt that I received when I inquired on another board was:

Open Bolt usually means a bolt with a fixed firing pin. The gun is fired with the bolt open. Pulling the trigger releases the bolt to drop on the round, igniting the primer. Very basically, a gun that is carefully and purposely designed to slamfire. Whereas a Closed Bolt gun has a moving firing pin. The bolt is closed, pulling the trigger releases a hammer which strikes the firing pin, igniting the primer.

That sort of helped me understand it, but the additional info in this thread is requiring some more thought.

May 12, 2007, 11:10 PM
What's the benefit?

Simplicity mainly. Pulling the trigger releases the bolt. Goes into full auto fire. Release the trigger to catch the bolt, stop it from going forward.

May 12, 2007, 11:22 PM
It is generally simpler to design a trigger mechanism for an open bolt firearm than for a closed bolt one. For instance the trigger mechanisms of the Bergmann and the Stens. Therefore cheaper to manufacture.

May 12, 2007, 11:27 PM
Open bolt systems don't really help with cooling during firing. After the bolt is held open (finger off trigger), it A) allows air to circulate through the barrel to cool more efficently and B) Keeps the next round out of the chamber.

B is important when you generate a butt load of heat during firing. If you stick another round in there right away, the previously created intense heat can potentially "cook off" the next round. This is why you see open bolt systems applied mostly to FA weapons. If they were closed bolt systems, and they started to cook off rounds, the weapon would fire untill it's ammo was gone.

May 12, 2007, 11:30 PM
If they were closed bolt systems, and they started to cook off rounds, the weapon would fire untill it's ammo was gone.It takes awhile for a round to cook off--seconds at least. It seems improbable that successive cook offs would allow a rate of fire that would keep the gun hot enough to make the process self-perpetuating.

On the other hand, even a single unexpected discharge can be disastrous.

May 12, 2007, 11:36 PM
So, it's kind of like a controlled slam fire? And I assume the bolt has an extractor? What's the benefit?

For an open bolt subgun or machinepistol, it essentially is a controlled slamfire. With an open bolt rifle caliber machine gun, like many beltfeds, its a little more like a very precisely timed sequence of events where pulling the trigger lets the bolt slam shut while keeping the firing pin at bay just until the bolt is in battery. The effect is essentially the same, but the timing becomes very important (unless you want to go searching for pieces of your face, that is...).

The benefit of an open bolt subgun is...well, simplicity of design and manufacture. They generally have less parts than closed bolt systems and are easier to manufacture. Also, because of the mass of the bolt slamming home they're generally very reliable. For a open bolt machinegun, like a beltfed, the benefit is really in cooling. During the firing sequence there's no difference, but after you stop firing (perhaps scanning for another target) with the weapon at the ready, the bolt remains open allowing cool air to enter the barrel and cool things down, hopefully to a point where the barrel won't melt or you won't experience a cookoff.

44 AMP
May 13, 2007, 02:37 AM
AN open bolt machine gun (like the M60) cannot have a cookoff (unless it has a broken firing pin). I had this very clearly explained to me by a Marine Gunery Sgt at Small Arms Repair School USAOC&S.

The majority of SMGs are open bolt, and have a fixed firing pin. The H&K MP5 is different, firing from a closed bolt.

The 1921 & 1928 Thompson SMG fires from the open bolt, but does not have a fixed firing pin. Wartime production pressure turned the 1928 into the M1, and then the M1A1, which does have a fixed firing pin.

Yes, (pistol caliber) in open bolt SMGs, it is a form of controlled "slam fire".

The reason they are called open bolt is that the bolt is open (to the rear) when you pull the trigger, and locks to the rear after the firing cycle ends, leaving the chamber empty and open to the air. This system is seldom found in semi autos, as the weight of the bolt slamming forward to fire tends to make accurate shooting difficult. Until the Govt stopped it, you could buy TECs, MACs, Cobray, UZI, and other semi auto open bolt SMG replicas as they were legally pistols.

The M2 .50 caliber Browning machine gun can be operated either in open or closed bolt mode. Normal mode is open bolt, to aid in cooling. Fired semi auto (single shot) from a closed bolt the M2 is capable of surprising accuracy with a good barrel.

A really interesting automatic rifle is the WWII German FG 42. 8mm Mauser, and it fired from the closed bolt in semi auto mode, and from the open bolt when fired full auto!

The manual of arms for an open bolt gun is exactly opposite (backwards) from a closed bolt gun. With a closed bolt gun (AK,AR, FAL, etc.) the gun is hot with the bolt forward (closed) and a mag in place, and cold (safe) with the bolt open.

With an open bolt gun (Thompson, Mac, Uzi, MP 40, etc) the gun is hot with the bolt back (open) and a mag in place, and cold (safe) with the bolt forward (closed).

Hope this helps.

May 13, 2007, 07:54 AM
As I haven't yet fired my open bolt carbine, but am preparing to, this has been an informative thread. The gun itself came with nearly zip documentation.

May 13, 2007, 11:51 AM
An overlooked reason for the open-bolt system as used in pistol caliber firearms is the ability to use:
1) A lighter bolt, or
2) A lighter recoil spring-

Essentially, the rearward thrust of the round being fired has to stop the forward progress of the bolt first, then begin moving it to the rear. This would be in the fixed firing pin class of subguns, BTW, nothing more sophisticated. Someone here will probably remember the correct terminology for this process, but whats happening is the round is actually firing as the bolt is still slightly in motion forward. PDI, IIRC?

nagib otayek
May 14, 2007, 11:24 AM
1) In the open bolt weapons, usually submachines that fire pistol cartridges like the famous "Grease Gun" the Bolt is usually heavy and the recoil spring quite tough, enough to hold the bolt forward, so no gas is capable of escaping back until the bullet has left the barrel. The bolt has a built in firing pin that comes in contact with the primer, once the bolt has stripped a round and chambered it; in this moment the continuous pressure of the forward travelling bolt, ignites the primer, the bullet leaves the barrell and so as long as the trigger is held the gun keeps cycling until its empty or you release the trigger.
2) In machine guns and assault rifle that have selectors, the firing pin is floating in its channel and controlled by devices that differ from one type to another. To make it simple imagine a 1911 gov. model in which you can switch the disconnector from on to off. In one position it would fire full auto, in the other it fires semi-auto. In WWII, Polish officers used to change the firing pin of the Radom semi auto pistol and instead install a longer one. If captured by the enemy they used to surrender the handgun with the slide fully drawn :eek: back and the magazine charged, when the german soldier held his arm toward the pistol, the Polish officer released the slide and the pistol went firing in full automatic mode. Just for History.