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Posted: 4/15/2006 8:04:53 PM EST
This has caused me confusion in the past - does it mean that the firing pin strikes the moment the bolt locks the cartridge into the chamber, as opposed to being UNABLE to fire until the trigger is pulled AFTER the bolt locks into the chamber?

Obviously, a machine gun (or any other kind of gun) can't fire when the bolt is partially open, or it would fire out of battery, right? What am I missing here?
Link Posted: 4/15/2006 8:06:30 PM EST
The bolt still locks before ignition, but the bolt rests in the back position, and only travels forward when the trigger is pulled.
Link Posted: 4/15/2006 8:07:23 PM EST
Open-bolt MGs lock the bolt open when not firing, as opposed to closed-bolt firearms like we're all familiar with where the bolt rests closed and in battery when not firing. Chambering, firing, and returning back to the open position constitutes one cycle of the action.
Link Posted: 4/15/2006 8:07:49 PM EST

Originally Posted By Combat_Jack:
The bolt still locks before ignition, but the bolt rests in the back position, and only travels forward when the trigger is pulled.




the fireing pin is fixed also
Link Posted: 4/15/2006 8:08:18 PM EST
With machine guns that fire from an open bolt you don't have to worry about cooking a round off in the chamber, because it is empty.
Link Posted: 4/15/2006 8:09:05 PM EST
which means that it's easy to take a file to a few parts, and make the gun full-auto. which is why the ATF controls them as MG's, and almost every semi-auto firearm today uses the closed-bolt mechanism of operation.
Link Posted: 4/15/2006 8:10:37 PM EST

Originally Posted By MuRDoC:

Originally Posted By Combat_Jack:
The bolt still locks before ignition, but the bolt rests in the back position, and only travels forward when the trigger is pulled.




the fireing pin is fixed also



Not always. Common on SMG's but not always the case with LMG's.

It could be the port talking , but I'm pretty sure that the MG-3 (or MG-42/59) LMG fires open-bolt, but does NOT have a fixed firing pin.

However, the Husqvana SMG definitely has a fixed firing pin, as does the Swedish M45B (and I assume the SW 76 which is based on it) and probably their precursor, the Sten gun.

Link Posted: 4/15/2006 8:11:07 PM EST
[Last Edit: 4/15/2006 8:23:49 PM EST by U_Down_Wit_OPP]
Others are correct about the basic function.

It's also worthy of note that most all open bolt designs used a disconnector to prevent "runaway" full auto function, while the "sear" (be it unique or integrated with the trigger assembly) actually mitigates the shot cycle.

As a result, they are very, very simple to convert to FA by simply filing down, snapping off or removing this disconnector, leaving the actuation of the sear itself to moderate the bolt travel (which itself dictates the actual ignition of the rounds).

Part of the reason that grandrathered Title 1 open bolt MAC's sell for $1000 is because they're the closest thing you get to a legal machinegun without having to fill out Form 4's. People buy them as "emergency machineguns" that are legal in their grandfathered configuration, but are a file away from being a fully functional SMG should they ever "need it".
Link Posted: 4/15/2006 8:11:23 PM EST

Originally Posted By fossil_fuel:
which means that it's easy to take a file to a few parts, and make the gun full-auto. which is why the ATF controls them as MG's, and almost every semi-auto firearm today uses the closed-bolt mechanism of operation.



Yes.
Open bolt firearms are also less accurate, but also simpler to make and design.
Most "crude" SMG designs are open bolt - Sten, Sterling, Mac, MP40, M3A1, Sterling, etc.

The MP5 is a closed bolt SMG, which is part of the reason it is so accurate.
Link Posted: 4/15/2006 8:11:45 PM EST
Also it *should* help to cool it down without having clear it, lock the bolt back in a firefight or many mag dumps at the range
Link Posted: 4/15/2006 8:12:21 PM EST
Machine gun barrels can get real hot, hot enough for a round to cook off (ignite spontaneously from the heat).

So, for safety reasons it’s best not to have a round in the chamber of a machine gun for any length of time. “Open Bolt” means that the weapon’s bolt is retracted and locked back when ready to fire. When you pull the trigger the bolt releases, closes, locks (except blowback of course), fires, and cycles normally. When you release the trigger the bolt stops in the rear position.

Needless to say this adversely affects accuracy some, but machine guns don’t have to be tack drivers.

The open bolt also aids in cooling.
Link Posted: 4/15/2006 8:13:53 PM EST

Originally Posted By fossil_fuel:
which means that it's easy to take a file to a few parts, and make the gun full-auto. which is why the ATF controls them as MG's, and almost every semi-auto firearm today uses the closed-bolt mechanism of operation.



I still have one of the open bolt RPB semis
Link Posted: 4/15/2006 8:19:11 PM EST
[Last Edit: 4/15/2006 8:21:44 PM EST by U_Down_Wit_OPP]
I've always been hesitant to buy one.
Shoot them with mild ammo and they will go FA.

It's the kind of gun that if for some reason you get pulled over with it, you stand a 50/50 chance of the cop pissing all over himself, figuring he caught someone with a "machinegun" and your gun winds up taking a trip to the ATF tech branch; and they aren't above testing open bolt semis with progressively lighter and lighter loads until the bolt cycle doesn't engage the disconnector and the gun burps off a burst.
Link Posted: 4/15/2006 8:21:30 PM EST
The M-16 fires from a closed bolt, but the M-16 based M-231 port firing weapon (used for tanks and Bradley's) fires from an open bolt. It also has no sights.
Link Posted: 4/15/2006 8:22:13 PM EST

Originally Posted By DK-Prof:

It could be the port talking , but I'm pretty sure that the MG-3 (or MG-42/59) LMG fires open-bolt, but does NOT have a fixed firing pin.



As they say, in vino veritas.
Link Posted: 4/15/2006 9:11:10 PM EST

Originally Posted By Thuban:
“Open Bolt” means that the weapon’s bolt is retracted and locked back when ready to fire. When you pull the trigger the bolt releases, closes, locks (except blowback of course), fires, and cycles normally.



My recollection is a little fuzzy, but I also seem to recall something about a round being chambered at some point during that process...
Link Posted: 4/15/2006 11:45:38 PM EST

Originally Posted By DK-Prof:

Originally Posted By MuRDoC:

Originally Posted By Combat_Jack:
The bolt still locks before ignition, but the bolt rests in the back position, and only travels forward when the trigger is pulled.




the fireing pin is fixed also



Not always. Common on SMG's but not always the case with LMG's.

It could be the port talking , but I'm pretty sure that the MG-3 (or MG-42/59) LMG fires open-bolt, but does NOT have a fixed firing pin.

However, the Husqvana SMG definitely has a fixed firing pin, as does the Swedish M45B (and I assume the SW 76 which is based on it) and probably their precursor, the Sten gun.


true, I should have said most
Link Posted: 4/16/2006 12:04:06 AM EST
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