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Subject: What does the US Army need in a next generation machine gun?
Herald12345    2/20/2008 5:04:22 PM
Or for that matter, what do the experts hewre consider would be achievable improvements in the general purpose machine gun? Since I am not an expert on this subject I'm here to learn. I will ask questions though. Its part of the educating Herald series. Herald
 
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WarNerd       3/3/2008 3:51:07 AM

Sure, there's a tech plateau on the ability to handle heat using steel. For ceramic materials we have just begun to climb the mountain.

I saw a slideshow on one of the NDIA smallarms conference lists talking about an extremely light barrel for a machinegun. The barrel had been created using a ceramic core and sheathed with I think titanium and could absorb enough heat that changing the barrel was not necessary at all.

You don't want to absorb the heat (unless the gun is water cooled), you need to dissipate it.
 
What they have done is designed a gun barrel that will not rupture or strip out the rifling when firing while white hot.  The ceramic also has lower conductivity for heat than steel so more heat passes out with the propellant gases.
 
This also means that the inner surfaces of the barrel and chamber will get hotter and stay hotter longer, so problem with cook-offs will increase.  An automatic weapon with a ceramic barrel will probably have to fire from an open bolt.  That would cost accuracy in a rifle, but not in a machine gun because most already are designed that way.

 
Almost all technology is like that, you make a series of subjective trade-off's in hope of a producing a superior product and call it 'optimization'.  Then the users complain about how some aspect of the older design was better while ignoring the advantages that are claimed for the successor.
 
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Ispose    MG Cooling Issues   5/21/2008 11:22:43 AM
Just Thinking out loud here but how about a Perforated shroud around the barrel with some sort of sponge type material surrounding the barrel. You can just pour water over the barrel, the sponge would absorb it, and as the barrel heated up it would dissapate some of the heat. Of course the "Sponge" would need to be heat resistant and you would only need to wet it during periods of sustained firings. Just a thought. The Assistant gunner can pour water on the shroud from his canteen if necessary.
 
 
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WarNerd    MG Cooling Issues    5/22/2008 4:21:48 AM
Just a few possible problems:
-   If dry, the sponge acts as an insulator, causing the barrel to overheat quickly.
-   The weight and balance of the weapon is continuously changing as water is added and evaporates.
-   If the cooling effect is not uniformly distributed you can warp the barrel.  Particularly if you fire the gun dry, then pour water in the top of the sponge.
-   If the barrel gets hot enough to generate steam, the steam will be trapped by the sponge and push the water away from the barrel, producing a sort of 'vapor lock'.
 
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Ispose    Re: WarNerd   5/22/2008 12:37:44 PM
You're right about the Steam problem..darn it. How About a High Strength Ceramic Barrel about 2" Diameter with Cooling Fins...Would look Buck Rogerish but should dissapte heat quite well. Weight shouldn't be much more than a Heavy Steel barrel. I'm thing this is for a Crew served, Tripod mount, M60/M240 Replacement. Not sure if it would be feasible for a SAW replacement. Fun to speculate.
 
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WarNerd       5/23/2008 4:59:38 AM

You're right about the Steam problem..darn it. How About a High Strength Ceramic Barrel about 2" Diameter with Cooling Fins...Would look Buck Rogerish but should dissipate heat quite well. Weight shouldn't be much more than a Heavy Steel barrel. I'm thing this is for a Crew served, Tripod mount, M60/M240 Replacement. Not sure if it would be feasible for a SAW replacement. Fun to speculate.


Ceramics are like concrete, they have good compressive strength, poor tensile strength, and lousy elongation.

The ceramic part of the barrel is generally only a liner.  Most designs then have a titanium or hyper alloy tube for dimensional stability and rigidity (droop) and to produce a more even temperature distribution, then many windings of graphite-epoxy tape for hoop strength (so that the barrel does not expand from internal pressure during firing). 
 
The graphite-epoxy layers do not conduct heat as well as metal and will be difficult to mount fins on without compromising their mechanical integrity.  Not impossible to do, but the additional heat dissipation from fins may not be significant.
 
The fins will also complicate barrel changing and be easily susceptible to damage.
 
 
 
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k3n-54n       5/26/2008 9:44:28 PM
not sure if I like the sponge idea, but if it were made of high density, open cell, aluminum foam, I can see it working OK.
It would be light, improve barrel cooling when dry, and work great when soaked.  put some kind of rain gutter underneath it to capture water that falls through so you could drop it on top again. 
 
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WarNerd       5/29/2008 4:33:42 AM

not sure if I like the sponge idea, but if it were made of high density, open cell, aluminum foam, I can see it working OK.
It would be light, improve barrel cooling when dry, and work great when soaked.  put some kind of rain gutter underneath it to capture water that falls through so you could drop it on top again. 


Nice try, but it won't work that way.  The "high density, open cell, aluminum foam" is not open enough to promote air flow, but does reduce the cross section for heat transfer so it will have an insulating effect.  Also aluminum is non-wetting so that it will not "soak" up water.
 
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flamingknives       5/29/2008 4:44:29 PM
If you want to wick heat away, aluminium and copper are the ones to go for. As copper is pretty heavy, aluminium is the better bet.

Something like filament wound graphite reinforced aluminium with a hard metal liner would be pretty good in terms of low weight and good thermal conduction, if you could only find a way to transmit it to a heat sink or radiate it away.
 
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Nichevo       6/8/2008 7:57:43 PM
How about packing it in something like steel wool?  Or Al of course. 

Also Al-plated carbon nanotubes with enormous surface area.  Perhaps hollow with coolant.

How about some sort of coolant pak with every box of ammo?  A capsule of liquid nitrogen perhaps, vented at intervals.

Maybe it would be simpler to use the bullets/casings as (even more of) a heat sink?

A coaxial fan for forced air cooling?

More efficient liquid cooling?  Flash evaporation?  NASA is using it.

 
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andyf       6/17/2008 10:04:29 AM
ok some off the wall ideas,
 are aerospikes applicable to bullet shapes?
can we apply the peltier effect for cooling barrels?
how long does the barrel need to be? if we make it shorter then   will it absorb less heat from the firing?
how about the gast gun principle- youd need 2 barrels but they could maybe be shorter
i think some sort of drum magazine  say like that calico mag maybe could help
 
 
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