AR15.Com Archives
 M231 Firing Port Weapon
von_landstuhl  [Team Member]
12/20/2006 7:04:17 PM
A photo of SPC Ross McGinnis, recently nominated for the Medal of Honor.

He has a M231 Firing Port Weapon with him in the Humvee. Anyone know how popular they are with non-Bradley troops?

ETA: I know they work in full-auto only, but what does it mean to "fire from an open bolt."

Dawg180  [Member]
12/20/2006 7:29:02 PM
It means that instead of the bolt cycling and closing after firing a round the bolt assembly stays in the open position after firing a round. This is typically done to allow more air to flow through the chamber and cool the weapon faster. On the M231 it is necessary due to the ridiculously high rate of fire (1200 RPM) and the lack of semi.

Most the pics of the M231 in service are either form the mid to late 1980's (and attached to Bradleys) or seem to be pics from the last year or so and are being used by Abrams crews.

IIRC, Abrams crewmembers are only issued 1 or 2 M16's or M4's per vehicle, or at least that was the way it was a few years ago. My guess is that the crews are appropriating them from storage so they have some sort of close range weapon. Although it doesn't have iron sights, I imagine at point blank range of a few yards the M231 and a 30 roudn magazine is adequate for chopping down anyone who tries to get in close to the tank and plant an IED or soemthing of that sort. Better than an M9 pistol at the very least.
Liberian_Gun_Smiff  [Member]
12/20/2006 7:55:21 PM
They are by far the rarest variant of the M16 weapons familia.

The only other 5.56 M15 that I am aware of that fires open bolt is the Colt light machinegun system.

On an open bolt system the firing pin is normally just a protrusion machined into the bolt face, as soon as the bolt "gets home" it pops the cherry and gets blown back again. The trigger holds the bolt BACK when released, rather than holding the hammer back as it would on a closed bolt system. It's really the symplest of designs, but normally is only used on pistol caliber weapuns like the Ingram M-10, or Sten.
Liberian_Gun_Smiff  [Member]
12/20/2006 7:58:32 PM
On an open bolt gun, you can fire a burst, let off the trigger, look into the ejection port and SEE the empty chamber and the magazine with a round on top. THIS IS THE READY POSITION! When it is ready to fire, the chamber is empty.
Black-Tiger  [Team Member]
12/20/2006 8:07:11 PM




ridiculous rate of fire? You betcha! Is almost as fast as a MAC-10, but spitting 5.56mm ammo downrange. Definitely somethign you want to hold on with both hands.
LoneWolfUSMC  [Member]
12/20/2006 8:12:53 PM
It is also necessary to prevent a runaway gun. Chambers on light machineguns can get hot enough to light off a round when it's chambered.
Liberian_Gun_Smiff  [Member]
12/20/2006 8:15:42 PM

Originally Posted By Liberian_Gun_Smiff:
On an open bolt gun, you can fire a burst, let off the trigger, look into the ejection port and SEE the empty chamber and the magazine with a round on top. THIS IS THE READY POSITION! When it is ready to fire, the chamber is empty.

That looks longer than I remember.........
I thought they were like 8"?

OH fuck! I met to quote black-tiger....
JBowles  [Team Member]
12/20/2006 8:20:37 PM
Spray and Pray Baby!!!
greco  [Member]
12/20/2006 8:38:34 PM
Been a long time since I fired one, but they are usually used with tracer rounds so you can see where the hell you're shooting. Originally fit in a mount on the side of an APC, you installed the gun in the vehicle mount by a push and turn action, like a bayonet mount.

By the way, not all open bolt guns used a fixed firing pin. The M60, MG42, MG34, FG42, and many others come to mind. They use an internal striker (the firing pin is attached to a spring activated operating rod and is released to fire a round) takes the place of a hammer..

The little Colts are cool guns though, if you gotta shoot RIGHT NOW, not much can top them.
dave89iroc  [Member]
12/20/2006 9:40:38 PM
these are quite interesting, are they still gas operated, or are they blowback?
1saxman  [Member]
12/20/2006 10:28:58 PM
No gas tube required to blow back open bolt. Think Tommy gun.
jvicent  [Member]
12/20/2006 11:46:05 PM
I carried one on top of a Bradly in Mogadishu. They're nice to have in a trench too, where sand has the nasty habit of falling sqaure into your ejection port. From 10 yards, from the hip, a burst into an E-type is no sweat if you practice a little. I remember a 1SG named Malloe who was pretty good with one. We started training with them to clear trenches just before Desert Storm, and the concept held on until we started losing them out of the inventory.

The firing pin on the M231 isn't fixed, like the one on an M3 Grease Gun. 3 concentric recoil springs and a really short guide rod. Fight like a mental patient to get one together if you weren't able to get the blasted cap on the tube just right.

Out of the back of a Brad, they're crud. ALL tracer, vacum hose on it with the brass bag. I guess it's better than letting someone walk right up one you though. Then again, we didn't sit still without dismounting.
gaspipes  [Member]
12/21/2006 1:05:22 AM

It's really the symplest of designs, but normally is only used on pistol caliber weapuns like the Ingram M-10, or Sten.


Just a minor point, but most machine guns fire from an open bolt. Sub gun or not.
mstennes  [Team Member]
12/21/2006 1:17:15 AM
o what if someone wanted to build one, would they be shit outa luck?
Phoebus  [Team Member]
12/21/2006 1:38:44 AM

Originally Posted By mstennes:
o what if someone wanted to build one, would they be shit outa luck?


An exact one, that fires from the open bolt, I'd say yes, SOL.

Every once in a long while you see some furniture from one in the EE, but it's pretty damn rare.
Ekie  [Team Member]
12/21/2006 1:50:27 AM

Originally Posted By 1saxman:
No gas tube required to blow back open bolt. Think Tommy gun.


They use a gas tube, are gas operated. Blow back and open bolt are two different things. For a AR to be both 5.56 and blow back it would have to weigh 15 pounds are so because of the massive bolt needed.
Phoebus  [Team Member]
12/21/2006 1:53:49 AM

Originally Posted By Ekie:

Originally Posted By 1saxman:
No gas tube required to blow back open bolt. Think Tommy gun.


They use a gas tube, are gas operated. Blow back and open bolt are two different things. For a AR to be both 5.56 and blow back it would have to weigh 15 pounds are so because of the massive bolt needed.


+1, although I'd love to see one that was. It would be hilarious. And then firing it? Whew! Imagine a truly blowback M231 at 1200 RPM....
Ekie  [Team Member]
12/21/2006 2:00:11 AM

Originally Posted By Phoebus:

Originally Posted By Ekie:

Originally Posted By 1saxman:
No gas tube required to blow back open bolt. Think Tommy gun.


They use a gas tube, are gas operated. Blow back and open bolt are two different things. For a AR to be both 5.56 and blow back it would have to weigh 15 pounds are so because of the massive bolt needed.


+1, although I'd love to see one that was. It would be hilarious. And then firing it? Whew! Imagine a truly blowback M231 at 1200 RPM....


That would be pretty exciting. Kalashnikov built a blow back 7.62x39 rifle in the 1940's, apparently it was quite a chunk.
Black-Tiger  [Team Member]
12/21/2006 2:06:35 AM


The particular port gun shown above doesn't have the collapsible wire stock that was common on them. I have looked all over the place, but have not found the complete buttstock assy., just the stock itself.

I've fired the weapon while in service back on 1993, the little bastard is fast and it will go through a 30 round magazine quick. It was fun to shoot.
sic_ness  [Member]
12/21/2006 2:39:42 AM
Theoretically, could an industrious GI put an m4 or a2 stock and buffer tube on it if he really wanted to?
Liberian_Gun_Smiff  [Member]
12/21/2006 3:43:26 AM

Originally Posted By gaspipes:

It's really the symplest of designs, but normally is only used on pistol caliber weapuns like the Ingram M-10, or Sten.


Just a minor point, but most machine guns fire from an open bolt. Sub gun or not.

Really?

As far as I know no Browning was ever OB. The old Colt belt feds I think was CB? M60 I can't recall? I think H&Ks are CB (rolling block, or some shit).


Wich ones are OB?
TonyRumore  [Member]
12/21/2006 8:31:37 AM
I had a complete retractable buttstock for one awhile back and had it installed on my M16A1 for awhile.

However, the buttstock could not be used on my A2 style lower. The reinforcements around the buffer tube got in the way of the stock retracting.

Tony

Dave_A  [Team Member]
12/21/2006 8:36:47 AM

Originally Posted By Liberian_Gun_Smiff:
They are by far the rarest variant of the M16 weapons familia.

The only other 5.56 M15 that I am aware of that fires open bolt is the Colt light machinegun system.

On an open bolt system the firing pin is normally just a protrusion machined into the bolt face, as soon as the bolt "gets home" it pops the cherry and gets blown back again. The trigger holds the bolt BACK when released, rather than holding the hammer back as it would on a closed bolt system. It's really the symplest of designs, but normally is only used on pistol caliber weapuns like the Ingram M-10, or Sten.


Correction:

Open bolt operation is normally only used on FULL AUTO WEAPONS, especially GENERAL PURPOSE MACHINEGUNS.

The reason is that OPEN BOLT OPERATION allows the BARREL TO COOL FASTER (since when the gun is not firing the barrel is open to air circulation...

Every MG in service today (except the M-2 50cal ) is open bolt - be it the PKM, MG-3, M240, or M249... I believe the RPD is open bolt as well...
GHPorter  [Team Member]
12/21/2006 9:12:00 AM
Nobody so far has mentioned the rate of fire for the M231. It's like a bazillion rounds per minute-the system has both a short action and a very fast lock time. AND it has no sights; it was designed to fire 100% tracers (out of that 8(?) inch barrel!) A very FAST bullet hose is what the M231 is!
Phoebus  [Team Member]
12/21/2006 9:43:38 AM

Originally Posted By Liberian_Gun_Smiff:

Originally Posted By gaspipes:

It's really the symplest of designs, but normally is only used on pistol caliber weapuns like the Ingram M-10, or Sten.


Just a minor point, but most machine guns fire from an open bolt. Sub gun or not.

Really?

As far as I know no Browning was ever OB. The old Colt belt feds I think was CB? M60 I can't recall? I think H&Ks are CB (rolling block, or some shit).


Wich ones are OB?


Almost all of them. All of the current US machinegun small arms except for the M2 are open bolt (the M249, M60, and M240).
CB1  [Team Member]
12/21/2006 9:55:23 AM

Originally Posted By Black-Tiger:
www.autoweapons.com/photos06/nov/381portg.jpg

The particular port gun shown above doesn't have the collapsible wire stock that was common on them. I have looked all over the place, but have not found the complete buttstock assy., just the stock itself.

I've fired the weapon while in service back on 1993, the little bastard is fast and it will go through a 30 round magazine quick. It was fun to shoot.


None of the M231s I saw from 1984-1987 had a wire stock. I didn't think they were issued beyond the inital trials.

Oddly enough, the operator's manual I have dated Oct 1982 still shows the wire stock. I was told that due to the high rate of fire they did not want us to use them dismounted (at the time) so they removed the stocks. Heck, in the 3 years I was a Bradley crewman, I fired one only once.

we were not allowed to install them (to prevent bent barrels while driving through trees) so they took quite a beating, always bouncing around the back of the vehicle.
mstennes  [Team Member]
12/21/2006 10:48:01 AM
I was thinking it would be cool to have the upper and furniture and build up a replica.
I remember that untill now the only pictures I saw of them they had the wire stock. If one could find all of the furniture, upper and wire stock could the stock be adapted to fit a A2 lower? I think I may have "found" my next build. Glad this topic came up as I had forgotten all about these little jewls.
45er  [Member]
12/21/2006 3:18:07 PM
45er  [Member]
12/21/2006 3:22:34 PM
Found a TM for one on ebay too:

cgi.ebay.com/Submachine-Gun-5-56-MM-Port-Firing-M231_W0QQitemZ150026778298QQcmdZViewItem
clutchsmoke  [Team Member]
12/21/2006 4:33:54 PM
I don't think open bolt operation has been explained clearly so I'll give it a shot.
An open bolt weapon really just slam fires.
The bolt is held back by the trigger sear and a magazine is inserted into the weapon.
When the trigger is pulled the bolt is released and moves forward.
It strips a round out of the mag, pushes it into the chamber and fires it.
The bolt will keep cycling like this until the trigger is released, holding the bolt back or until you run out of ammo.
One could take for example a tommy gun and remove all the trigger components and still make the gun fire.
Just pull the bolt back and hold it, insert loaded mag and let the bolt go.
The gun would fire at it's normal rate and run until the mag ran dry.
HardShell  [Team Member]
12/21/2006 4:40:12 PM

Originally Posted By Black-Tiger:
... The particular port gun shown above doesn't have the collapsible wire stock that was common on them. I have looked all over the place, but have not found the complete buttstock assy., just the stock itself...


You mean like this?:





fadedsun  [Member]
12/21/2006 5:02:26 PM

Originally Posted By HardShell:

Originally Posted By Black-Tiger:
... The particular port gun shown above doesn't have the collapsible wire stock that was common on them. I have looked all over the place, but have not found the complete buttstock assy., just the stock itself...


You mean like this?:

i2.photobucket.com/albums/y12/HardShell/RWB%20-%20ARs/M231-2-small.jpg

i2.photobucket.com/albums/y12/HardShell/RWB%20-%20ARs/M231-1-small.jpg



How fast do you pull your finger to get 1200 TPPM?

HardShell  [Team Member]
12/21/2006 5:11:44 PM

Originally Posted By fadedsun:
How fast do you pull your finger to get 1200 TPPM?



Don't I wish!

I picked this up off of EE several years ago to try for a compact-as-could-be AR for my Miata (smallest vehicle I own, with the smallest trunk). I found that it wasn't really much more compact than a CAR and with a much worse cheek-weld (akin to an AK underfolder). I ended up using my Bushmaster ARM for the Miata's trunk instead and only kept the M231-style stock assembly as an oddity (as noted, you don't see many of them anymore). [Before you ask, SBRs aren't allowed in Alabama. ]
gaspipes  [Member]
12/21/2006 6:00:38 PM

Originally Posted By Phoebus:

Originally Posted By Liberian_Gun_Smiff:

Originally Posted By gaspipes:

It's really the symplest of designs, but normally is only used on pistol caliber weapuns like the Ingram M-10, or Sten.


Just a minor point, but most machine guns fire from an open bolt. Sub gun or not.

Really?

As far as I know no Browning was ever OB. The old Colt belt feds I think was CB? M60 I can't recall? I think H&Ks are CB (rolling block, or some shit).


Wich ones are OB?


Almost all of them. All of the current US machinegun small arms except for the M2 are open bolt (the M249, M60, and M240).


to add: MG42, MG34, FG42, MG3, PKM, L86A1, Type 67 GPMG,

practically all machine guns fire from an open bolt, to permit air cooling from the breech between bursts
2manytoys  [Team Member]
12/21/2006 7:17:24 PM

Originally Posted By GHPorter:
Nobody so far has mentioned the rate of fire for the M231. It's like a bazillion rounds per minute-the system has both a short action and a very fast lock time. AND it has no sights; it was designed to fire 100% tracers (out of that 8(?) inch barrel!) A very FAST bullet hose is what the M231 is!


Did you not read any of the previous posts? It has been mentioned a few times.
chewbacca  [Team Member]
12/21/2006 10:09:33 PM
Put an Aimpoint on the Carry handle and a M4 receiver ext and stock... OH YAH!
Hotgun  [Member]
12/21/2006 11:11:40 PM
Sorry, the M231 won't accept an M4 or M16 extension....the extension is filled with a much different set of springs and "stuff" to make it function. Yep, 1200 rpm.....and the proper load-out is 100% tracers.....hum, wonder where I came up with my theory for reduced barrel life with GMCS ammo?? Might have to mine out to "play" one of these days and get reaquainted. I'd rather not shoot one, though...much prefer the M4A1.

Hotgun
BushmasterGuy77  [Member]
12/21/2006 11:34:37 PM

Originally Posted By Black-Tiger:
www.autoweapons.com/photos06/nov/381portg.jpg

www.autoweapons.com/photos06/nov/381portf.jpg

ridiculous rate of fire? You betcha! Is almost as fast as a MAC-10, but spitting 5.56mm ammo downrange. Definitely somethign you want to hold on with both hands.


[Arnold] You are one ugly Motha Fucka [/Arnold]
Russ4777  [Team Member]
12/22/2006 2:02:39 PM

Originally Posted By sic_ness:
Theoretically, could an industrious GI put an m4 or a2 stock and buffer tube on it if he really wanted to?



No. The carrier and buffer springs are quite different in the 231 FPW. The carrier uses a thing inserted in the back called a Striker that actually acts like a hammer. The buffer spring we are used to in a conventional AR is replaced by a triaxial spring assembly.

The thing an be fairly easily converted to an M16 / M4 configuration to slow down the rate of fire and to use easily available parts. Of course, it then fires from a closed bolt. 231 FPW parts are really hard to find.

I am a test engineer for BAE Systems (formerly FMC then United Defense). We designed and build the Bradley. I have had the opportunity to fire the FPW both from a ramp ball port and off-hand. It's kinda fun to shoot but generally useless with it's VERY high rate of fire.
GHPorter  [Team Member]
12/22/2006 7:27:09 PM

Originally Posted by clutchsmoke:
An open bolt weapon really just slam fires.
No, not really. A slam fire is not the same as the way most open-bolt weapons work; the majority use some kind of hammer system or a striker. A slam fire is an undesired firing of a cartridge as it's chambered. Most MGs have some delay (albeit small) between closing the bolt and actually firing the cartridge. For example, the M60 MUST close its bolt before the striker can possibly move forward enough to whack the firing pin. On the M231, things are "complicated." See below:

Originally Posted By Russ4777:
The carrier uses a thing inserted in the back called a Striker that actually acts like a hammer.
MUCH better to have a striker than a fixed firing pin ESPECIALLY with rifle ammo.
OLY-M4gery  [Team Member]
12/22/2006 7:31:58 PM
2 things spring to mind



Aimpoint

and

Beta C Mag

AR15_Man  [Member]
12/22/2006 8:25:13 PM
Any pictures of the Buffer Assy would be great, this buffer tube would make a Pistol build a little shorter..
DocMike  [Member]
12/22/2006 9:52:08 PM
Food for thought.



Doc
clutchsmoke  [Team Member]
12/22/2006 9:56:46 PM

Originally Posted By GHPorter:

Originally Posted by clutchsmoke:
An open bolt weapon really just slam fires.
No, not really. A slam fire is not the same as the way most open-bolt weapons work; the majority use some kind of hammer system or a striker. A slam fire is an undesired firing of a cartridge as it's chambered. Most MGs have some delay (albeit small) between closing the bolt and actually firing the cartridge. For example, the M60 MUST close its bolt before the striker can possibly move forward enough to whack the firing pin. On the M231, things are "complicated." See below:

Originally Posted By Russ4777:
The carrier uses a thing inserted in the back called a Striker that actually acts like a hammer.
MUCH better to have a striker than a fixed firing pin ESPECIALLY with rifle ammo.


Point taken, but some open bolt weapons have a fixed firing pin on the bolt.
garand1945  [Member]
12/23/2006 2:58:01 AM
I thought of the open bolt was not to let the barrel cool, but to not keep a round in the chamber long enough to cook off. Because in the open bolt mode the cartridge it is fired almost as soon (we're talking split second) as it is chambered, it can't get hot enough to cook off. That's why GPMGs use it so that you can maintain sustained fire.
Phoebus  [Team Member]
12/23/2006 3:09:28 AM

Originally Posted By garand1945:
I thought of the open bolt was not to let the barrel cool, but to not keep a round in the chamber long enough to cook off. Because in the open bolt mode the cartridge it is fired almost as soon (we're talking split second) as it is chambered, it can't get hot enough to cook off. That's why GPMGs use it so that you can maintain sustained fire.


It's both. You want the barrel to cool in general, since machinegun barrels get hot fast.
Tumerboy  [Member]
12/23/2006 11:20:27 AM
Well in terms of making a replica, I've found the stock/buffer tube assembly, and a set of handguards. The main part I can't find is the gas block/mounting screw/whatever you want to call it.

Anyone know where to find one? Anyone have an original that would want to take some precise measurements so I could get one manufactured?

Thanks
GHPorter  [Team Member]
12/23/2006 12:13:02 PM

Originally Posted By clutchsmoke:

Originally Posted By GHPorter:
MUCH better to have a striker than a fixed firing pin ESPECIALLY with rifle ammo.


Point taken, but some open bolt weapons have a fixed firing pin on the bolt.
Yep, some do, but I don't know of any "formally adopted" rifle-caliber weapons that do. Some expedient weapons, maybe, but not weapons adopted for issue.

And the thing I wanted to stress is that a slam fire is effectively a malfunction that can have catastrophic consequences, while a properly operating open-bolt weapon is doing its job the way it was designed to.