A couple questions on Machine guns


September 26, 2003, 02:48 PM
Im sorry, this has been probably asked before, I am a little nave on this subject.

What is the difference between open bolt vs closed bolt? Advantages vs disadvantages?

What is the difference between a subgun and a machine gun? Does it have to do with caliber?

Any response from the full auto crowd would be appreciated.

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September 26, 2003, 02:54 PM
Open bolt: when the weapon is ready to fire, the bolt is locked to the rear. When the trigger is pulled, the bolt slides forward, chambering a round, fires, and is pushed to the rear to begin the cycle again.

Advantages: cheap and easy to make. Little chance of cook-offs since the only time a cartridge is in the hot chamber is when it's miliseconds for firing.
Disadvantages: the mass of the bolt moving forward doesn't lend itself to accurate fire.

Subgun: an automatic weapon chambered for a pistol caliber cartridge.

AJ Dual
September 26, 2003, 02:54 PM
Open bolt has a fixed firing nub on the bolt face, and is held rearward by the trigger/sear, and strips a cartridge out of the magazine, chambers it and fires all in one motion. It keeps cycling until the trigger is released, catching the bolt rearward once more.

The advantage is that it's easier to build/design, and the open bolt allows the barrel and chamber to cool fom both open ends. Also since cartridges are held in the mag until fired, there's no chance of cook-off in a hot chamber.

The disadvantage is that open-bolt guns often require heavy bolts, stiff, hard to cock recoil springs, and since the bolt jumps forward on firing wiggling the gun a bit, the first shot is not as accurate as a closed bolt design.

Closed bolt fires when the bolt is closed with a firing pin being struck by a hammer, much like most rifle designs. A cartridge can be held in the chamber until ready to fire.

The advantages are that the first shot, securely chambered is more accurate since until firing, the only parts movment is the trigger and the hammer falling. They can be made lighter since the bolt can be smaller as it locks into place against the reciever, or the chamber end of the barrel depending on the design.

The disadvantage is an unfired round can be cooked off since it's waiting in the chamber instead of the magazine, if a long burst has just been fired. It's also usually harder to design, and more complex to manufacture.

A submachine gun is usually defined by being chambered in a pistol cartridge, or at least one that's smaller than most rifle rounds. But there's some overlap, as some very short versions of cut down assult rifles are also sometimes considered subguns too.

Legally though, from an ATF, and U.S. private ownership standpoint, anything that fires more than one shot automatically with only one pull or holding of the trigger is legally considered a machinegun with all having the same regulations up to .50 caliber. (over .50 is a "destructive device" classified with rockets, shells, grenades etc.) There are no legal or registration distinctions between a full-auto pistol, subgun/carbine, rifle, or crew-served machinegun on a tripod. They're all treated the same.

Mike Irwin
September 26, 2003, 11:39 PM
Here's a little spot of trivia...

Some machine guns have been made, normally light MGs with shoulder stocks but also some submachine guns, so that when the selector is set to automatic fire mode they fire from the open bolt, with the bolt remaining open after a burst to allow air circulation.

With the selector set to semi-automatic mode the gun fires from the closed bolt position, like a rifle, to promote greater accuracy.

I'm digging to find a couple of specific examples, but I think the Bren might have been one.

September 27, 2003, 12:47 AM
I didn't know that. Very interesting.

September 27, 2003, 02:39 AM
Open bolt has a fixed firing nub on the bolt face,

Nope, nope, nope. Some do but not nearly all.

Most noticable is the 1921 & 1928 Thompson Submachine Gun, but there are many others.

September 27, 2003, 02:46 AM
Good question. Can't say as I've seen it here before. Or anywhere else for that matter. S'ok. Still a good question.
Open bolt is just that. The firearm fires from an open bolt. The bolt must be pulled back and locked open in order to fire when the trigger is pulled. Most submachine guns(smg) work this way. (The French made semi-auto Gevarm .22 rifle works on an open bolt too. Hard to come by though.) And yep, cooling is the reason. Not cheap manufacture though. The .45 Thompson was totally machined. Cost a bundle to make. Then they came out with a Thompson M-1. Less machining.
The early Thompson SMG's, the Sten gun and the Sterling smg all operate with an open bolt. They're also all chambered in a pistol round and all are full auto or select fire.(full or semi) The Uzi also operates with an open bolt but the H&K MP-5 does not. Nor does the M-16K. (nice 9mm version of the M-16. Nice and accurate)
The advantage of a closed bolt SMG is you don't have the weight of the bolt moving back and forth over the point of balance of the weapon. This is why you hear about the Thompson climbing when you fire it. A closed bolt smg is inherently more accurate than open bolt smg. Think of a semi auto rifle. You pull the trigger and it goes bang. With an open bolt, you pull the trigger and you wait for the action to close before it goes bang. Your sight picture could change in that time.
A machine gun uses the same round or bigger as the standard issue rifle. Up until Korea. There are 3 weights of MG's. Light, medium and heavy. The BAR and Bren gun are light MG's. The .30 Browning and .303 cal Vickers gun are medium MG's. The .50 Browning, the 12.5 and 14.5 Russian MG's are heavy MG's. The M-60 is a medium mg but it did not use the same ammo as the battle rifle of the time.
The M-16 is not an MG nor is it an SMG. It's a different battlerifle. They get called 'assault rifles' but no troopie get issued a different rifle when he's told to assault something. A real assault rifle used a round that was the same calibre, but a cartridge less than a standard battle rifle but with the same punch.
There are all kinds of books you can get in the library about firearms. Read all of them. Get Hatcher's Notebook. He was there for all the developement of the semi-auto rifle and he could write.

September 27, 2003, 03:01 AM
Wasn't that the FG-42 that did that? (NOT the MG-42, but the squad support weapon German paratroopers carried in WW2, it fired from an open bolt in full auto mode, and from the closed in semi, )

(It held a 20 round magazine, and IIRC fired a 8mm Kurz round? )

September 27, 2003, 09:50 AM
The Madsen M50 is another open bolt subgun without a fixed firing pin.
Standard equipment was a spare firing pin stored inside the grip.

September 27, 2003, 10:08 AM
One thing you dont hear much about with the open bolts is the safety issues. A closed bolt for the most part just like a semi gun with the round chambered and the safety on. It loads and unloads the same. To safely load an open bolt gun, the bolt is forward, or closed on an empty chamber, and a magazine is inserted and the safety is set or locked. This is the only safe way to carry it. To make the gun ready, you pull the bolt back to the open or "cocked" position. Now to someone who is only accustomed to a closed bolt gun, this looks to be safe, as even if the bolt goes forward and chambers a round, it wont(normally) fire until the trigger is pulled. This isnt the case here. When the bolt goes forward on the open bolt gun, it will fire. It doesnt even have to make it all the way to the rear and catch the sear while cocking it. If your hand slips while drawing it back, it can still strip a round and fire. If you drop it, even with the bolt down on an empty chamber, if the safety(most have one) isnt on locking the bolt, the gun can fire. Once the gun is cocked, if you want to make it safe you have to remove the mag FIRST, then lower the bolt then reinsert the mag. If you pull the trigger to lower the bolt without first removing the mag, it will fire, as its supposed to. Most open bolt guns have a safety notch on the tube beyond the "cocked" point that you can slip the charging handle into to render it "safer" than just on cocked, some dont and have a more traditional switch type safety, but this still isnt the best way to make it safe and your much better off lowering the bolt. These things are a ton of fun, but you do need to familarize yourself with a few things before you start to play. :)

Mike Irwin
September 27, 2003, 11:01 AM

You might be right. I haven't found the book yet that discusses this. I haven't really looked for it, either. :)

OK, the Bren fires from an open bolt. Big warning in Small Arms of the World about that fact...

September 27, 2003, 11:51 AM
General curiosity:
-I live in IL (-a no-FA-no-silencers-no-AOW-no-nuthing-no-how state), but suppose I started a gun factory in a state that allowed FA/silencers/whatever properly registered. Can I make brand-new full-auto firearms anymore? Who can buy them? I have read a couple different pages attempting to explain the 1986 (?) ban, "dealer samples" and so on, but get lost in the details....

September 27, 2003, 11:57 AM
You can get all of the appropriate licenses and permits and manufacture NFA weapons, but it will be expensive just setting up. Machineguns can be made for sale only to law enforcement, government or export. They cannot be sold to civilians.

The '86 ban is essentially a ban on the manufacture of new machineguns for sale to civilians. All machineguns registered prior to the ban are still legal and can be bought/sold. Those made after the ban cannot be sold to civilians.

The '86 dealer sample is basically a dividing line for dealers. A dealer must have an official letter from some agency requesting a demonstration before he can aquire dealer samples. If the sample was made before the 1986 ban and the dealer closes his business, he may keep the sample. If it is a post-86 sample, he must transfer the gun upon closing his business.

The other NFA items can be manufactured and sold to any qualified individual or agency. In fact, you don't even need to be a manufacturer to make them. Fill out ATF Form 1 (one set forms and $200 for each item to be made), get it approved and you can make NFA weapons (excluding machineguns) at home.

September 27, 2003, 01:29 PM
All these terms have meanings that are in dispute and are used incorrectly by many. This is the way I understand and use the terms:
Submachine gun. Full auto or select fire weapon that fires a pistol caliber cartridge.
Battle Rifle: A rifle that fires a full sized rifle cartridge. It may be select fire, or it may be bolt action or semi-auto. Full auto fire with a battle rifle is usually not practical or effective.
Assult rifle. A select fire weapon that fires an intermediate sized cartridge that is neither a full sized rifle round or a pistol cartridge. It is also usually of intermediate size; larger than a sub-gun and smaller and lighter than a battle rifle in terms of the weapon itself.
Machine gun: A weapon that is larger than a battle rifle that fires a full sized rifle round or bigger and is either select fire or full auto. The machine gun is usually a crew served weapon and is usually belt fed.

This is just my take on the subject and as I noted, it is subject to inturpretation.

September 27, 2003, 02:04 PM
The FG42 fired the full house 7.92x57 Mauser.


September 27, 2003, 02:47 PM
Thanks guys, that cleared things up for me:)

Mike Irwin
September 27, 2003, 11:09 PM

Is that an actual FG42?

Is it yours?

If so, you have QUITE the treasure. Very few are held by private individuals.

Last one I saw for sale was asking nearly $50K.

September 28, 2003, 12:10 AM
No, it's not mine. I wish it were real and mine.

It's one of those $1500 non-firing repros that were made a few years ago. I believe they were made from parts kits.

Mike Irwin
September 29, 2003, 02:17 AM
Found one reference to a LMG that operates from the open bolt when set to automatic, and closed bolt when set to semi...

The M1941 Johnson.

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