"Firing from an open bolt"


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Lone_Gunman
July 15, 2009, 07:28 PM
Can someone explain this to me as it applies to full autos?

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PTK
July 15, 2009, 07:30 PM
Firing pin is fixed forward, bolt is held back - when you squeeze the trigger, the bolt goes forward. Just before the bolt is fully forward, the firing pin hits the primer and ignites the cartridge. The bolt goes fully forward (the last few thousandths of an inch) and the bullet exits the barrel. The pressure backwards pushes the bolt back all the way, extracting and ejecting the spent case. The cycle, in a straight full auto, continues until the trigger is released or the ammo runs out.

Lone_Gunman
July 15, 2009, 07:32 PM
OK, so as I understand it the Thompson submachine gun is an open bolt design. How was it carried by GI's? Did they carry it with a magazine in, and the bolt locked rearward, so all they had to do was pull the trigger to cause the bolt to close and fire? or did they carry it with magazine in, and bolt forward?

With the firing pin fixed to the bolt, is an open bolt design more prone to firing out of battery?

PTK
July 15, 2009, 07:34 PM
It depended - what conditions? Front line? Patrol? Simply moving from A to B? Although many open bolt SMGs are considered "drop safe", I wouldn't call the Thompson one of them. Generally, unless use was expected very soon, they would be carried with the magazine in and the bolt forward, safety on. This also kept dirt and debris out of the gun.

To fire, the bolt handle was pulled back, the safety was turned off, and the trigger was depressed.

MikkOwl
July 15, 2009, 07:39 PM
Also noting something that is perhaps obvious: there's never any cartridges in the chamber of open bolt type weapons. They only enter for a fraction of a second to be fired and then be instantly extracted.

The idea is to prevent cooking off the cartridge in the chamber and avoid inadverted discharges. Everything cools better too.

Also, I'm pretty damned sure the Thompson SMG was a closed bolt design, only changed later in regards to make it cheaper to mass produce.

The downside of open bolt constructions is that safety can be worse (jolts and impacts can potentially make many of these types of weapons release the bolt and fire a round). Accuracy is worse, as the bolt has to move forward and jam a cartridge in before firing, moving the sights a bit. And lastly, lots of opportunity for dirt to enter and foul through the open ejection port.

EDIT: Pretty much all machine guns are open bolt aren't they? MG42 was, and I think all the current M240 and M249.

M60
July 15, 2009, 07:40 PM
The M60 machinegun is an open bolt design. It is carried with a belt in the tray with the bolt back ready to fire except for the safety. If the trigger ever fails to catch the bolt in the rearward position, you get what is called a "run away gun" that will not stop firing till it runs out of ammo.

-Mark.

Acera
July 15, 2009, 07:45 PM
Open bolt designs are the simplest and one of the most reliable designs, but it does come the cost of accuracy. That heavy bolt mass moving forward after the trigger is pulled makes it harder to hold on target. Not that is a big problem, just a nuisance that has to be compensated for.

There is even an open bolt design for the AR family. It was used in the firing port weapons made for the Bradleys, had a very high cyclic rate of about 1200 rpm. They had no sights, you looked through a vision block (periscope) and used all tracers to walk your bullets on to target, in theory. Of course we only had 30 round mags, and it used them up in about 1 1/2 seconds. Trained to suppress RPG teams with them.

The Lone Haranguer
July 15, 2009, 07:48 PM
To illustrate the difference, stop firing in the middle of a magazine by taking your finger off the trigger. In an open bolt system, the bolt (thus the firing pin) is held to the rear, the chamber is empty and the top round in the magazine is protruding up into the action, ready to be stripped off by the bolt when it returns forward upon your pressing the trigger again. Examples: the above mentioned Thompson, M3 "grease gun," Uzi. In a closed bolt system, the bolt is forward with the top round in the magazine stripped off and chambered, only waiting for the firing pin to be released by your pulling the trigger. Examples: M16, H&K MP5.

With all the mass of the bolt essentially "slam firing" the gun at a rapid rate, open bolt submachine guns can be quite a handful to control. The closed bolt MP5 is much easier to shoot.

Gryffydd
July 15, 2009, 07:56 PM
With all the mass of the bolt essentially "slam firing" the gun at a rapid rate, open bolt submachine guns can be quite a handful to control.
Wouldn't lock time suck as well?

Deanimator
July 15, 2009, 08:00 PM
Just before the bolt is fully forward, the firing pin hits the primer and ignites the cartridge.
That's "advanced primer ignition" (API). Many open bolt submachineguns work that way, but they don't have to. It allows the bolt to be lighter since the cartridge case has to overcome not just the inertia of the bolt and the force of the recoil spring, but also the forward momentum of the bolt, before it can start to back out of the chamber. Many blowback submachineguns don't use it, particularly guns with hammers and floating firing pins.

AK103K
July 15, 2009, 08:27 PM
Closed bolt guns are what most people are familiar with, as most (there are a few open bolt SA guns around) semi autos are a closed bolt gun, just minus the select fire capability.

The open bolts while simple, tend to be more of a safety issue for people not familiar with them. First and foremost, they "look" safe to a someone familiar with SA guns, as the bolt is locked back and the action is open. With an open bolt gun, its "safe" (of sorts) when the bolt is forward on an empty chamber. Just the opposite of what most are used to.

Most of them have some sort of lock on the bolt that locks the bolt forward, in hopes of stopping an accidental discharge, due to dropping or something bumping the charging handle. Some work better than others. The majority of open bolt guns are not really drop safe. They can also be a problem if you start to pull the charging handle to the rear, and your hand slips. All it takes, is for the bolt to move rearward just far enough to strip a round off the mag, and the gun will fire if the bolt goes forward.

Another safety issue is putting the gun into a "safe" condition. In order to do so, you need to lower the bolt, which usually requires pulling the trigger. Of course, you need to drop the mag to do so without the gun going off. You'd be surprised how many people just pull the trigger. Some guns do have a safety notch in the receiver, usually at the rear, like an MP40, that the charging handle slips into. In that case, the bolt is open, but is not being held by the sear. To release it, you pull up and out and ease the bolt down until the sear picks it up.

Once you get them down, the open bolt guns are very shootable. The bolt moving forward before the shot is disconcerting to a lot of people until they get used to it. The open bolts usually dont offer that first shot, or longer range accuracy that the closed bolt guns do, but they still are very shootable guns. Many of them also dont offer select fire, and are full auto only.

amprecon
July 15, 2009, 10:23 PM
A Thompson was my first and only full-auto experience. It takes a little to get used to, but by the third magazine it was "rock-n-roll". As with any weapon, you learn it's quirks and adapt yourself to it.

Tommygunn
July 15, 2009, 11:35 PM
. Generally, unless use was expected very soon, they would be carried with the magazine in and the bolt forward, safety on.

The safety on the Thompson only works when the bolt is back. It turns a cam into a cut-out in the face of the sear, and the sear will only take it while it's in the upward position, and it will only be there when the bolt is back.

Also, I'm pretty damned sure the Thompson SMG was a closed bolt design, only changed later in regards to make it cheaper to mass produce.

No, the Thompson was always an open-bolt design. In WW2 they tried to expedite production and lessen the expense of producing it by removing the complicated Blish delayed-blowback device, simplifying the rear sight, eliminating the grooves on the barrel and leaving off the Cutts Compensator. This was the M1 model. The later M1A1 model eliminated the hammer & firing pin and made it a "slam-fire" gun by simply milling a "nub" on the face of the bolt as a "firing pin."

Acera
July 16, 2009, 12:29 AM
Well Tommygunn, it seems like you are living up to your handle. Nice bit of information.

peyton
July 16, 2009, 12:53 AM
The M3 and M3A1 "Grease gun", was in the army inventory up until the 1990's. I was easy to work on and was truly a "bullet hose". You could see the .45's flying to the target. It was carried by tank crews, and APC drivers. The dust cover has a bracket that locked the bolt when it was closed.

natman
July 16, 2009, 03:05 AM
Wouldn't lock time suck as well?

Sure does. Not only is the bolt heavy compared to a firing pin, but it gets delayed stripping off the top cartridge in the magazine and feeding it into the chamber. So lock time is a LOT longer than it would be with a closed bolt.

Also with a closed bolt gun the trigger only has to deal with enough spring force to ignite the primer. The rest of the recoil energy is handled by the main spring. In an open bolt gun the trigger has to deal with a spring strong enough to handle ALL the recoil energy, equal to the firing pin spring and the mainspring combined. This makes for a heavy and creepy trigger pull.

Open bolts are great for full auto and terrible for semiautos. There have been several open bolt semiauto 22s; the Winchester 55, the Marlin 50, the BSA Ralock, the Gevarm E1, the Voere 2005. None have been remotely successful.

Cohibra45
July 16, 2009, 03:37 PM
After shooting a Thompson back a few years ago, I still want one and when I hit the lottery, it's one of the first things I plan on getting!!!!!!!!!!

Heck of a lot of fun!!!!!!!!:evil:

1KPerDay
July 16, 2009, 05:41 PM
Another possibly unintended consequence of open bolt designs, at least in submachine guns, is that the cyclic rate can be very fast. My M11/9 runs at about 1000-1100 rpm, and quite a bit faster with a suppressor. Some guys with polished internals, mags, bolt buffers, suppressors, etc. have gotten theirs running up to 3000+ rpm. :eek:

There are several variations of "slow-fire" conversions for this reason. Some people like to slow them down to 600-750 rpm for more accurate (and inexpensive) fire.

Corey
July 16, 2009, 05:45 PM
Actually, firing rate has nothing to do with if it is an open or closed bolt design, it has to do with reciprocating mass (bolt weight), bolt travel, and spring rate. This applies to blowback or delayed blowback designs. If gas operated, then you can add port pressure to the mix.

rcmodel
July 16, 2009, 05:46 PM
The dust cover has a bracket that locked the bolt when it was closed.
And well it should have.

One of the most dangerous things about some of the simplest open-bolt SMG's was the danger of leaving the bolt closed on an empty chamber when you had a magazine in it.

Smack the butt on the grond while parachuting, going prone, of just drop one and the heavy bolt could open enough from it's own weight to pick up a round from the mag and fire it.

They were actually safer with the bolt open (cocked) when you had a mag in the gun due to the very heavy trigger pull.

rc

AK103K
July 16, 2009, 05:57 PM
I would say the majority of the open bolt guns I've shot had pretty nice triggers, and some were downright light. My buddies MP40 is one of those. You really have to pay attention when your shooting it.

While some like the MAC's have a pretty quick rate of fire, I think most are more realistic, and guns like the M3/M3A1 are down right slow. With the MAC's, I usually get 3-5 round bursts with an average trigger pull, most of the others, I can usually get one shot "bursts" from them without any troubles, and easily make good 50-100 yards shots with them.

ACP230
July 16, 2009, 08:53 PM
I know some folks who can do very accurate shooting with open bolt subguns.
I can do fairly accurate shooting with them.
Lock time may be longer than on closed bolt guns but I don't notice the difference when shooting them.
I shot a PPSH (open bolt) and a Reising M50 (closed bolt) on the 4th of July. The PPSH had a lot faster ROF, but it and the Reising didn't feel much different in the time the actions took to fire.

MikkOwl
July 16, 2009, 09:35 PM
The issue is more when trying to do accurate single shots. The heavy bolt moving forward and other stuff happening as you are already struggling to control your trigger will affect your aim. Not very relevant for close range, but for longer ranges I imagine it matters. Some of these weapons can't even do single shots.

Deanimator
July 16, 2009, 11:23 PM
Some of these weapons can't even do single shots.
The M3 and M3A1 only fire full-auto, but the rate of fire is so slow that it's not hard to firing single shots through trigger manipulation.

fireman 9731
July 16, 2009, 11:37 PM
Man... the things you can learn in an evening here on The High Road :)

cowart
July 17, 2009, 11:55 AM
Pretty much all machine guns are open bolt aren't they?The Heckler & Koch MP-5 is a conspicuous exception

Gryffydd
July 17, 2009, 11:56 AM
The Heckler & Koch MP-5 is a conspicuous exception
Except isn't it technically not a machine gun?
*As in small arms classifications, not government regulations.

Deanimator
July 17, 2009, 11:59 AM
Except isn't it technically not a machine gun?
*As in small arms classifications, not government regulations.
No, it's a machinegun by any reasonable definition. It fires two or more consecutive rounds with a single pull of the trigger, whether it's full-auto or (n) round burst.

Submachineguns are just a sub-category of machineguns.

Deanimator
July 17, 2009, 12:01 PM
The Heckler & Koch MP-5 is a conspicuous exception
A lot of early rifle caliber and larger machinegun designs fired from a closed bolt, including the Maxim, Vickers, Schwarzlose, Revelli, Browning M1917/M1919, Browning .50 (M1921/M2), etc.

pryd4tsmg
July 18, 2009, 04:43 PM
Speaking of lock time & Thompsons, a couple friends and I spent an afternoon on the leavee utilizing the rear sight to it's max extent. All the fields were tilled, and impacts showed up really well. I was curious to see how well it would shoot out to 600 yds. and beyond, and was surprised how accurately it could hit. Lock time was lousy, but you could compensate for it when you got used to it. Full Auto at that range was laughable. But for single shots with the sight elevated as high as it would go, we could have made an enemy soldier pretty nervous. Raise gun, fire, lower gun, wait for impact. (Now read that real slow and you get the idea.)
Had a boss who wanted to shoot it, and at thirty feet and 20 rounds, didn't even hit his hardhat. Same gun, different shootist. But I do love the Family Thompson!

Avenger29
July 18, 2009, 08:15 PM
Some may be wondering "Why use an open bolt design"?

Well, not only is it very simple to manufacture, but it helps with cooling, too. You don't want a round cooking off in a hot chamber.

ACP230
July 18, 2009, 08:43 PM
Folks with good reflexes can fire single shots from some subguns you wouldn't think would allow it.
My son can even fire singles from a PPSH.

I can do it from a Swedish "K" for the first hour of a shooting session.
After that I get tired and two and three round bursts is what I get when I try to shoot "short."

Last time I fired a Reising M50, .45 ACP closed bolt subgun, I got one single shot with the gun set on FA. I think that's the first time I've been able to do that.

Hk91-762mm
July 18, 2009, 08:59 PM
The original UZIs were open bolt ,,I bought a cut up one hoping to make a semi auto out of it-- Too Much work,
Same with the port Said I got =Muffler pipe with a fixed fireing pin bolt on a spring.. I have seen the STEN guns --same thing
PPHS Chi-com Is an open bolt too...
Funny-you should heve seen me trying to remove the fireing pin from the Port Said - :neener: Yeaa I can laugh now !!

Deanimator
July 18, 2009, 10:03 PM
Same with the port Said I got =Muffler pipe with a fixed fireing pin bolt on a spring..
The Port Said's just a copy of the Carl Gustav (Swedish K).

Sunray
July 18, 2009, 10:31 PM
"...sure the Thompson SMG was a closed bolt design..." Nope.
"...all machine guns are open bolt aren't they?..." Nope.
"...technically not a machine gun?..." Nope. Sub-machinegun.
"...dust cover has a bracket that..." The dust cover was/is also the safety.

Wile E Coyote
July 19, 2009, 03:41 AM
Add to the open-bolt design the ole M-60 that I lugged around for a spell in the early 1990's. That one was easy enough to single fire. Heck, even expected to do so during a portion of certain range qualifications.

graywolfls
July 19, 2009, 04:13 AM
Just a side note. I have a Demro 45 cal open bolt semi. It is a delight to shoot, uses grease gun mags and even as a semi auto it fires so fast most people think its a full auto. Its got a lock built in (combo /sesame type) And the words THINK in bright red on the rear of the receiver, Gives one pause before you send a 45 cal stream of bullets down the range. To the best of my knowledge this is one of the very limited open bolt semis out there, but they do show up every now and then.

justashooter in pa
July 19, 2009, 02:03 PM
Open bolt sub guns gained favour in the years following the First War. Their design was refined over the the next 3-4 decades in the interest of enhanced reliability, reduced cost, simplified manufacture, and reduced weight, until they fell out of favour in the 60's. A good example of the development sequence is the Beretta design series. Guns in this series from the 20's and 30's were complicated and required hundreds of machining steps. They reached a peak of complexity in the 1938 model (which was well regarded), which had a floating firing pin that was levered into action by a pivot as the bolt came home on a cartridge. By '42 the floating firing pin had been replaced with a fixed pin, and excess weight like the barrel shroud and compensator were gone, as well. the 1951 model was a stripped down 42/43 with a wire stock ala grease gun, but never really caught much attention. Procurement was already looking to get out of machined receivers, entirely, and to replace pistol cartridges with "intermediate" cartridges like the 8X33/762X39/etc.

Deanimator
July 19, 2009, 02:26 PM
Add to the open-bolt design the ole M-60 that I lugged around for a spell in the early 1990's. That one was easy enough to single fire. Heck, even expected to do so during a portion of certain range qualifications.
A lot of gas operated guns fire from the open bolt. That's why they weren't popular as synchronized aircraft guns. The lock time was long enough to make precise synchronization very difficult. On the other hand, they made good flexible guns, the Lewis, Vickers "K", and ShKas being examples.

Voerelover
July 19, 2009, 08:22 PM
Try the openbolt voere mod. 2005 smg in .22lr you can write your name on a pine board! fires about 600 rds being a .22 recoil is just not a problem I picked two guns back in 1988 at Atlanta SWD. Hard to find now but they are still around, and it is no doubt the cheapest Mg openbolt gun around and add loads of fun!

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