Technical: 'fires from open bolt' ?

[Orsogato]

What exactly does this mean. I know the original BAR, Sten, Mp 40 etc. use this method of operation but how exactly does this work?

How does the pin strike the primer if the bolt is open? I have never actually fired one of these weapons and am somewhat confused at this terminology.

[paratwa]

An Uzi is an open bolt weapon. When you cock it the bolt is back. A simple description is they slam fire. When the bolt comes forward it contacts the cartridge and then firing pin hits the cartridge. The bolt is forced back and in a semi-automatic stays back till the trigger is pulled again. This is a simple description of what occurs. I am sure someone with more experience with open bolt weapons could give you a more technical description.

[Tex]

The open bolt weapons I'm familiar with have a fixed firing pin so when you pull the trigger the bolt slams closed firing the cartridge.

[sturmgrenadiere]

Open bolt firing weapons are pretty simple, generally. Sten guns, later Thompsons, I think MP38/40s, and a whole host of other sub-machine guns fired from the open bolt, especially with non-high power cartridges like the 9mm para and the .45ACP, to name two. As described above, the action is simple. Take the STEN gun. The bolt is locked back and held until released by the trigger. As the bolt travels forward under the power of a spring, it strips a round from the magazine, then it pushed the round into the chamber. Curiously, the bolt does not then lock in place, like an AR or an FAL. It just slams shut against the breech face. On the bolt face, instead of a firing pin as you might imagine (one that is a seperate piece that is hit by the hammer once the round is chambered), there is a little nipple or "pin" machined into the face of the bolt, so that as the bolt slams the round into the chamber, it then, as the bolt seals the chamber, ignites the primer. The recoil of the round firing sends the bolt back to either relock in place, if a semi-auto, or to compress the recoil spring until it sends the bolt back forward to repeat the process in full auto. However, unlike a locking bolt that is engineered not to unlock until the round has had the peak of the pressure curve impart it's force against it, on an open bolt, newton ensures that the bolt starts traveling backwards as soon as the round starts down the barrel. However, bolts on open bolt guns tend to be heavy, for a reason. Their weight helps determoine the cyclic rate of fire, and also it helps make sure the round is out of the barrel before the case is fully withdrawn, so you don't get a lot of gas blow out the chamber.

This action is incredibly simple, easy to mass produce, and relatively maintenace free. Again, just look at the STEN gun. Ugly, rough, made in the hundreds of thoudands with minimal machine work and tooling, but still a full auto 9mm SMG.

Now, on the flip side, most true machine guns fire from the open bolt. M240G, M249 SAW, M2, etc... The main reason I have been told they are open bolt weapons really has to do with the fact that after you run a belt or two though them they get very hot and you don't want to leave a round chambered, as there is potential for it to cook off and if the machine gun fired from the closed bolt, you could get a run away gun. Plus, just as critical, it aids in cooling (leaving the air able to circulate out the barrel and action with the bolt back, verses having the air circulation closed off with a bolt forwards. These factors are important for barrel life and the frequency of changing barrels during sustained fire.

Open bolt guns are not as "accurate" in that there is some amount of movement of the weapon once the trigger is pulled due to the inertial mass of the bolt and action lurching forward. But when you are dealing with a ten round burst, what is match grade accuracy...

[Templar]

*Edit-Sturmgrenadier did an excellent job with his post, but I was typing my response while he was posting his. We've got some good people on this board! http://www.fnfal.com/forums/biggrin.gif*

Open bolt systems keep the bolt locked back when the weapon is cocked. The cartridge is not in the chamber either, it remains in the magazine. When the trigger is pulled, the bolt is released, strips the round out of the magazine, and then closes on the round in the chamber where the firing pin contacts the primer, setting the round off.

It is basically a controlled slam fire.

Many fully automatic submachine guns, squad autos, and belt feds especially were and are engineered to fire from an open bolt. It helps prevent "cook offs" from occuring because the round is not in the chamber until the trigger is pulled and won't sit in a super hot chamber after a long burst of automatic fire.

It is also much easier to design an open bolt sub gun than it is a closed bolt sub gun. The only closed bolt sub guns that I know about are the HK MP5, the Chilean S.A.F. SIG copy, the Steyr TMP, the IMI Mini UZI (which is marketed as having the option of closed bolt function) and the Colt CAR-15 9mm. I'm sure that there are others that I haven't listed. All the others are open bolt.

There are disadvantages to open bolt designs as well. Accuracy suffers greatly because you have a very heavy bolt that slams forward every time that you pull the trigger, which causes the front end to nose dive. This is one reason MP-5's are much more accurate, generally, than an UZI. Also, because you have to carry the weapon with the bolt cocked, safety becomes an issue. If dropped, the bolt could slam forward, firing the weapon. The Germans had a problem with this in the MP-38/MP-40. The UZI is a much safer design than that however.

Dust can also enter the weapon through the open bolt much more easily than a closed bolt system.

Anyway, howe that helps some.

[This message has been edited by Templar (edited December 20, 2000).]

[DABTL]

The one thing that is not touched on by the others is an open bolt gun has a somewhat startling delay on firing the first round that you must become accustomed to in order to be accurate. It feels somewhat like shooting a flintlock with the delayed ignition. You pull the trigger and wait for the bolt to move forward. The first time it happened I thought the gun was not functioning at first. Then it went off. Startling first experience.

[vfrdirk]

In response to Bill's comment:

I actually like that delay. It IS a little disconcerting at first, but I got used to it pretty fast. My experience with open bolt weapons consists of the M60 and the UZI. In both cases, the delay was long enough that I could release the trigger in time to pull off doubles and in many cases, singles. (Singles are VERY easy on the UZI)

Although I only had the opportunity to fire standard 9mm through the UZI, I would have liked to try some of the Izzy service 9mm which, I understand, is quite hot.

Dirk

[Krieger]

Originally posted by Orsogato:
What exactly does this mean. I know the original BAR, Sten, Mp 40 etc. use this method of operation but how exactly does this work?

How does the pin strike the primer if the bolt is open? I have never actually fired one of these weapons and am somewhat confused at this terminology.
The reason why full-auto weapons are designed to fire from an open bolt is to aid in cooling and help to prevent a condition known as "cook-off". This happens when a chambered round goes off by itself because of excess heat. In an open bolt weapon there is never a round in the chamber until you are actually firing.