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Old August 24, 2012, 11:09 PM   #1
GunQuest
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M249/M240 Belt Fed Open Bolt Specific Question

Hi Guys,

My first post so I hope I don't get flamed. I have been searching the web and youtube for answers but cant seem to find the exact answer.

I am working on a digital project that involves very specific and accurate weapon mechanics so in order to stay true I am trying to get every detail right.

Question is this: After the M249 or any other belt fed MG runs dry (end of belt, no more rounds) the reload is as follows (safety not considered, just mechanical): lift the cover, insert new belt, close cover now this is where I am confused, since it is an open bolt rifle does the bolt stay locked to the back after the last round? I presume it does, to aid cooling and prevent cook off. If it is to the back and the new belt is in does the cocking handle need to be manipulated or is the weapon read to fire? Meaning will the forward motion of the bolt strip the round off the belt or does the rearward motion of the cocking handle load the round.

Any help would be appreciated.
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Old August 25, 2012, 12:39 AM   #2
44 AMP
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Ok, here you go, but please bear with me as I am going from memory and the last time I handled an M240 was 1978....And I'm too lazy to go find the right book,

First off, with an open bolt system, ANY open bolt system, odds are the bolt will be forward after it runs dry. And this is because of the user, not the design.

In an open bolt system, yes, the bolt will remain to the rear after the shot, if the trigger is released. IF the trigger is still held back on the last round, the bolt will go forward on an empty chamber. This is usually the first indication the shooter has that he is out of ammo. Full autos are like that.

Belt fed guns using the push through type links typically feed the belt on the rear ward movement of the bolt. The forward movement moves the feed pawl back to the "outside" position, so it snaps over the next round in the belt. The bolt going forward does this at the same time it strips the round out of the link, shoves it into the chamber and fires it.

The bolt coming back after firing extracts the fired case and ejects it, and advances the belt putting the next round into position to be fed into the chamber. The movement of the belt shoves out the empty link. If the trigger is held back, the cycle repeats automatically until the ammo runs out or the trigger is released.

If the bolt is on its way back when the trigger is released, it comes back, and is held there. If the bolt is on its way forward at the moment the trigger is released, it fires the round, and is caught and held when it comes back.

The position of the bolt when inserting the new belt is not important except that it is critical to the way you load the belt. You load the belt into the gun one way if the bolt is forward, and a differnt way if it is back. And if you use the wrong one, the gun will either not fire when the trigger is pulled, or jam when the bolt is pulled back.

There is a part of (or on) the feed tray called the cartridge stop. It sticks up, and "stops" the round from going past the position for feeding into the chamber. There is also a part, or parts known as "belt holding" or "retaining" pawls (they keep the belt from moving backward).

If the bolt is back, you load the belt with the first round up against the stop. Close the cover, pull the trigger and rock and roll.

If the bolt is forward, you load the belt with the first round just past the holding pawls, close the cover, and then pull the bolt back. This moves the first round into feeding position.

If you put the round all the way to the stop, with the bolt forward, pulling the bolt back will jam the gun. If you just hook the belt over the holding pawl(s) with the bolt BACK, sending the bolt forward will close it on an empty chamber.

This is a general description, common to a lot of guns. I'll have to look up a pic of the M240 parts to be certain, but I believe the 240 works this way, as well. When I do get able to check it out, I'll post a confirmation (or a denial).

The military will have a set procedure, to load the gun the same way each time (helps cut down on mistakes). While you can get a gun running by loading either way, they will teach one way only, such as only loading with the bolt back (pulling it back if needed, before inserting the belt). Gunners will be trained to that standard. We were taught to pull the charging handle back if/when the gun quit. Then open the cover, and insert a new belt to the stop.

Hope this helps. Welcome to TFL!

andI'm going to move this to the full auto forum,
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Old August 25, 2012, 01:21 AM   #3
GunQuest
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Wow what a detailed reply, more than I had hoped for. Thank you so much.

I think I get it now. In general the bolt will lock to the front after the last round unless by some magical timing the shooter releases the trigger as bolt is on its way to chamber/fire the final round which will send it back and with the trigger released it will lock.

Perfect, need to make some adjustments to my project but it is very clear now.
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Old August 25, 2012, 11:55 AM   #4
44 AMP
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Glad to be of service.

Long, long ago, in a places far, far away, I was a machine gun repairman for Uncle Sam. Can tell some stories about the guns and their foibles, if you are interested.....

again, welcome to the Firing Line!
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Old August 25, 2012, 12:17 PM   #5
DaleA
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Dang, I love this site.

Hey gunquest, you can get a chance to shoot the real thing, see how it works, and get practical experience using it in the field and get the bonafide manuals for it and everything, as well as a host of other neat weapons if you go down to the Post Office and ask to see the Army recruiter. Free food and clothes too! FTA=Fun Travel Adventure!

Last edited by DaleA; August 25, 2012 at 12:23 PM. Reason: added a note for gunquest
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Old August 25, 2012, 06:39 PM   #6
Crosshair
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Here is a good video showing the loading procedure for the M249.

http://www.youtube.com/watch?v=k1NozyVJ7js

I'm going to disagree with you a bit 44 AMP. From my understanding you NEVER NEVER NEVER reload a M249 with the bolt forward. You always retract the bolt before poping the cover.

The reason is that it is a bad habit to get into. If you pop the cover with the bolt forward after shooting, you potentially have the bolt forward on a dud round which could cook off while the cover is open.
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Old August 25, 2012, 07:52 PM   #7
44 AMP
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Crosshair, what I was describing was the way it could be done, mechanically.

What you are describing is the way it is to be done, practically!
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Old August 25, 2012, 11:38 PM   #8
Crosshair
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Ah, OK then.
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Old August 26, 2012, 04:11 AM   #9
GunQuest
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Thanks 44 AMP I would love to hear your stories.

Crosshair I did watch that video before I posted here, doesn't really explain the bolt operation, but thanks to 44 AMP I made some changes and now the function is correct.

I have another technical question. From my research the charging handle cannot be pulled back with the safety on if that is true when the safety is off and the charging handle is now traveling back can the safety be engaged? If so what happens to the handle, it appears to ride forward as long as the bolt is not locked back.

Also with the ejection port, it has a cover over it much like the m16 dust cover, however I am unclear on what forces it open. Is it the travel of the bolt to the rear or the front or simply the force of the spent casing pushing out?
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