Whats the point of an Open-Bolt?

[Pony Express]

Hello everybody,
I am curious as to what the advantages of firing from an open bolt would be. It only seems that it would cause problems. It translates into more time between when the trigger is pulled and when the primer is struck, as well as leaving the chamber and inner workings of the gun wide open to collect grit and grime, especially in a dusty climate. IIRC, the M60 and M249 fire from an open bolt, and it seems like that would cause lots of problems for our troops.

Can anyone enlighten me?

THanks,

 

[MCMXI]

Open bolt firearms run cooler with lower temperatures at the chamber which reduces the chance of a round "cooking off" as can happen with a hot chamber, a live round in that chamber and a closed bolt. If there are other benefits to open bolt designs I'd be interested in hearing all about them too. Accuracy has to suffer with an open bolt design for the reasons that you mentioned, particularly the increased lock time and the perturbation of the action/receiver. But for firearms where the rate of fire is more important than MOA accuracy I can see the benefit.

 

[DMK]

In the case of a subgun like the M3 Greasegun, it's also cheaper and faster to make.

 

[MCMXI]

Is there anything fundamentally different about the way that the bolts in open bolt firearms lock prior to firing? Do most of them use a rotating bolt?

 

[ArmedBear]

Note that open-bolt guns are "bullet sprayers" generally, not 1000 yard match/sniper rifles.

 

[Jubjub]

In full power machineguns, it aids cooling, and prevents cookoff, where the round in the chamber fires from the heat of previous firing. In pistol caliber automatic weapons, it makes for a very simple weapon with no separate firing pin, hammer, etc. Cheap and quick to manufacture. That big heavy bolt smacking the gun forward at the instant of firing also mitigates recoil a bit.

 

[Vern Humphrey]

That big heavy bolt smacking the gun forward at the instant of firing also mitigates recoil a bit.

It also delays bolt opening a smidgen. In a blow-back weapon like many submachineguns, you can use a lighter bolt with an open bolt mechanism.

 

[9mmepiphany]

the bolts in open bolt firearms lock prior to firing

what makes you think most of them lock?

most guns that fire from an open bolt are submachine guns and are blow back operated

 

[GunTech]

Actually, most open bolt sub guns use Advanced Primer Ignition (API). The primer is struck just before the bolt completes forward travel, so the round has to over come the forward momentum of the bolt. This is why open bold SMGs like the Sterling require much less spring power than closed bolt guns.

A well designed open bolt SMG has very little recoil recoil. Witness My 12 year old daughter firing my open bolt ZX-5

http://guntech.com/misc/smg.mov

 

[scythefwd]

The bolt design can be much simpler than that of a closed bolt design. I have been crawling through sand with my M249 and I have NEVER had a failure to fire/feed from it. It is an extremely reliable gun... until you try to feed it from an M16 magazine then it jams every 2 rounds. The 249 is actually used on point targets out to further distances than the M16 is as well. I was much more accurate with my SAW than I was with my 16. I got to where I could single shot it everytime and guarantee a hit out to 400y at the range... if there wasn't much wind. Yes, you can single shot a SAW, not it doesn't hurt the action (like many will say it does, but it is just letting the sear catch earlier)... it just isn't the role the rifle is designed to fill which is to put a wall of lead between you and a target.

 

[Maverick223]

It is cooler and much more simple (making if faster to produce and cheaper). I am not aware of a SMG that fires from an open bolt and uses any type of locking mechanism...I believe that all use blow-back. In contrast all of the MGs (that I know of) that fire from the open bolt use a gas operating system with a rotating bolt.

 

[1911 guy]

Cooler running, fewer parts to fail and yeesh, I'm getting tired. Forgot what I was going to type next. I'll sit and be quiet now.

 

[natman]

Yes, an open bolt has long lock time, a bolt slamming shut while firing and an heavy, creepy trigger pull. None of these enhance accuracy, but in a machine gun benchrest accuracy isn't a very high priority. Why fire a bunch of bullets into the same hole?

Advantages: no cook off, bolt is open after firing allowing the barrel to cool, sturdy firing pin machined into bolt face.

 

[PTK]

Actually, most open bolt sub guns use Advanced Primer Ignition (API). The primer is struck just before the bolt completes forward travel, so the round has to over come the forward momentum of the bolt. This is why open bold SMGs like the Sterling require much less spring power than closed bolt guns.

A well designed open bolt SMG has very little recoil recoil. Witness My 12 year old daughter firing my open bolt ZX-5

http://guntech.com/misc/smg.mov

Sterlings don't count, they have negative recoil!

 

[blackops]

I see open bolt firing as a simple way to decrease temperature. In which case most open bolts are going to be applied to full auto rifles that fire a significant amount of rounds in single spurts. Of course will never be as accurate, but more efficient of getting the lead out so to speak. The M249 SAW is a prime example of a successful open bolt firing system.

 

[gunnie]

open bolts ARE cheaper to mfg, but considering what a missle, jet fighter, chopper, air craft carrier cost... even a lousy humvee runs 120K. they have :

1--NO locking mechanism to jam up with mung and spooh.
2--above mentioned better cooling when not used, and no round in chamber too cook.
3--the simplest fire control group possible in a fully.
4--much higher cyclic rates than any closed bolt designs.

gunnie

 

[rcmodel]

1, 3, and 4 are not true if you apply it to rifle caliber & heavy machine guns.

All are more expensive and complex, do lock the bolt shut, fire from an open bolt, and have to employ rate reducers to slow down the RPM..

Subguns?
The H-K MP5 is one subgun that fires from a closed and locked bolt.
It uses a roller-delayed closed bolt and is one of the most controlable and accurate sub machineguns made in the world.

rc

 

[Maverick223]

Subguns?
The H-K MP5 is one subgun that fires from a closed and locked bolt.
It uses a roller-delayed closed bolt and is one of the most controlable and accurate sub machineguns made in the world.

That is one of the major factors in that particular SMGs accuracy.

 

[Vern Humphrey]

most guns that fire from an open bolt are submachine guns and are blow back operated

Correct.

Actually, most open bolt sub guns use Advanced Primer Ignition (API). The primer is struck just before the bolt completes forward travel, so the round has to over come the forward momentum of the bolt. This is why open bold SMGs like the Sterling require much less spring power than closed bolt guns.

Again correct. You can calculate the mass needed for the bolt of a blow-back weapon with the simple formula M1*V1=M2*V2, where M1 is the mass of the ejecta (bullet and powder gasses), V1 is the velocity of the ejecta, and V2 is the allowable velocity of the bolt in recoil. For centerfire cartridges, a heavy bolt is needed (which is why you see no blow-back .30-06s -- the bolt would have to weigh about 27 lbs.)

But if the bolt is moving forward at the time of ignition, the necessary mass is much reduced, because the forward momentum of the bolt can be subtracted from the final momentum in recoil.

 

[GunTech]

The H-K MP5 is one subgun that fires from a closed and locked bolt.
It uses a roller-delayed closed bolt and is one of the most controlable and accurate sub machineguns made in the world.

Unfortunately controllability is arguable. I've fired many MP5s as well as a large variety of open bot sub guns, and the MP-5 has one of the heaviest recoils of any 9mm SMG, thanks to its close bolt operation.

It is a nicely accurate 9mm carbine in semi, with no bolt jump to disturb aim. But it relative high cyclic rate and closed bolt operation makes it harder to control than 'primitive' guns like the Sterling, Swedish K, etc.

I can shoot my open bolt gun with one hand and put every round into a B-21 at 25 yards. I'd like to see that done with an MP-5.

Of course the MP-5 does look cooler and has more sexy add-ons.

 

[ny32182]

In the two weapons you mentioned, rifle caliber machine guns (M60/M249) I believe the applications are entirely based around heat: 1) No possibility of cook off, and 2) faster barrel cooling when not firing.

 

[Vern Humphrey]

In full-caliber machineguns, precision is not necessarily a virtue. A machinegun is more of a shotgun, and a modified choke is more useful in general applications than an ultra-tight full choke. Hence machineguns pay no accuracy penalty and gain lots of advantages from open-bolt operation.

 

[scythefwd]

Vern, Most machine guns were designed for grazing fire or suppressive fire... true enough. I have yet to shoot an M16A2 that was as accurate as my SAW. My zero groups were smaller when I was goofing around and doing single shots out of the SAW. The M16 can be made more accurate, but the military is looking for a hit on a man sized target out to 250m when qualifying (there are exactly 4 300m targets, all of which can be missed and you still can qualify as a sharp shooter). Hell for any non automatic rifle qual, you only have to shoot a little better than 50% (23/40) and they don't have any other criteria but hit/miss.