Do black barrels cool faster?

c1steve

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Has anyone, inc. the military, done tests on barrel colors? I would expect that a thin black coating would cool a hot barrel slightly faster, perhaps 10-20%, but that is just a guess.

I know there is a formula, which revolves around Black Body Radiation. However the formula uses degrees Kelvin (-273 C) it looks like one would need a barrel to be several hundred degrees hotter than ambient temp to have much of an effect.

Just wondering if there is cooling disadvantage with camo or green barrels as compared to black. Someone must have tested this, would be great to know before ordering my first GAP.
 

sotexhill

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Re: Do black barrels cool faster?

Good questions. I have not heard of any rigorous tests.

One can try touching the top of different color cars in a parking lot on a sunny day. A car has to be pretty close to white to not be warm/hot to the touch. I understand that heat gain and heat loss (radiation) are affected in the same manner by the surface. I've read where some racers paint their engine blocks and cylinder heads flat black for some increment of cooling. I doubt there is much difference between camo/OD and black.

Also you can check the temperature of any polished stainless steel trim in the parking lot.

Another factor might be that a dark non shiny barrel (black body) will pick up heat from the sun. So, looking at the TSRA high power match in central Texas on May 21, a hot day, the heat loss of a matte black barrel may have been offset by the heat gain in the sunlight. Barrels got pretty hot that weekend.
 

c1steve

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Re: Do black barrels cool faster?

I know there is a pretty big difference in engine cooling with a thin coat of black paint. This is in the shade, but at 180 degrees. So I would expect that there would some appreciable difference in the cooling of hot rifle barrels of different colors.

Regarding black vs. green, there is a huge difference. What happens is that in a warm state electrons orbiting atoms of the surface material are excited, and therefore fly much higher than electrons orbiting a cooler material.

For these electrons to drop down to a lower orbit, it has to lose a certain amount of energy to match any given orbit. If the color is a relatively pure color, the electron has to drop an extremely precise bit of energy, or it cannot do it. Therefore the cooling of a white or non-black object takes some time.

As black is all colors mixed together, the electron can lose any amount of energy and easily "find" an orbit that matches the energy it is losing. It does it over and over, until the object is at or near ambient temperature. If a pure color is mixed with a small amount of black, it becomes muddy to look at, but also exchanges heat much more rapidly than a color of similar darkness but has no black in it.

Therefore...I suspect that black will cool noticeably faster than camo. Dark green probably has some black in it, so would probably be better than desert camo in this regard.

A polished stainless barrel would be the worst. It is so smooth that the micro surface area is small. This is why tea pots were often polished, so they would cool down slowly. A bead blasted surface with a thin coat of non-gloss paint or anodizing would be ideal.
 

sotexhill

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Re: Do black barrels cool faster?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: c1steve</div><div class="ubbcode-body"><snip> A bead blasted surface with a thin coat of non-gloss paint or anodizing would be ideal.
</div></div>

How about when it is in the sun? Here in south Texas, that seems to be the norm.
 

Jer

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Re: Do black barrels cool faster?

Black absorbs the sun which makes black items HOTTER on sunny warm days. I tend to shoot on sunny and warm days so I don't see how a black barrel would stay cooler. In fact, if anything, it would likely cause my barrel to be hotter I assume. The exact opposite would be more likely to help keep it cool since it would reflect the sunlight. Personally, I'm not worried about it because we're talking about a few degrees here or there at most which is a drop in the bucket to what some barrels reach under normal operating conditions.
 

bohem

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Re: Do black barrels cool faster?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: c1steve</div><div class="ubbcode-body">
If the color is a relatively pure color, the electron has to drop an extremely precise bit of energy, or it cannot do it.

Therefore...I suspect that black will cool noticeably faster than camo. Dark green probably has some black in it, so would probably be better than desert camo in this regard.

A polished stainless barrel would be the worst.
</div></div>

True black bodies do not exist, but they can be closely approximated. To get an idea of how much heat will radiate from the surface you have an emissivity constant which is both material and coating dependent.

Also, the reason you have a specific amount of energy that must be released by an electron to drop orbits is because the electrons orbit along a mean pathway that is the average of a sine wave. They only move in discrete jumps because that's what it takes to have a continuous sine wave orbital path. This, however, has absolutely nothing to do with heat transfer.

Temperature is a measurement of the average kinetic energy of the moleculues/atoms in a material as they move/vibrate in whatever state they are in. The electron orbits are important for a chemical reaction, not a phase change or a temperature measurement.

As far as the actual heat transfer based on colors you're getting there. The barrel cooling question is conduction to the air, convection of the air, and radiation to the air, not just one component.

This was taken from <span style="text-decoration: underline">Fundamentals of Heat and Mass Transfer</span>, 5th ed. Incropera, DeWitt

<span style="font-weight: bold">Table A.11</span>
Black body emissivity constant = 1.0
Highly polished SS Ec = 0.17 (17% of the rate of a true black body)
Gold foil Ec = 0.02 (remember it's used for heat shielding in aircraft, race cars, space vehicles,etc.)

The difference between green coated AL and black coated AL are different, I won't quote the specific values because it's proprietary data for the specific application that I have sitting on my desk right now, but you're talking in the neighborhood of 20%

I don't have specific data for coated steels in front of me, but you get the idea.
 

FromMyColdDeadHand

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Re: Do black barrels cool faster?

Painted metal with film thicknesses over 1-2 mils (25-50microns) will have emissivities of about 0.90. Color is not a factor unless you are talking about highly filled metal flake films, which will lower the emissivity. The lower the emissivity the less and object is able to emit heat.

Get a barrel hot enough and the color might contribute to how fast it cools- but at those temps you'd have a hard time getting even a silicate system to be stable with inorganic pigments.

As to the sun's effect. White is the coolest (75% reflectance), black (5% reflectance) is the hottest- except if you are talking about IR reflective pigments that are used in IR camo and 'cool' building products. A 'cool' black will have 25% reflectance. They selectively absorb in the visible for color, but reflect in the IR for reduced heat build-up.

I've spent the last 10 years of my life on spectrally selective pigments for various applications, and black body radiation still drives me nuts.

But what would be coolest? A 1mil+ white paint? Reflectivity and emissivity all in one.
 

Jthoss0837

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Re: Do black barrels cool faster?

Use a gold barrel. (Joking)
Reflects the most radiant heat.
Ferrari uses a gold type of paint to coat the engine bay, to reduce heating
 

aj_brown

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Re: Do black barrels cool faster?

Some government testing was done and black and dark gray cooled the fastest. Testing was done on an indoor range so sun light was not a factor. Not every one shoots in the daytime. It was also found that coatings with ceramic in them acted as thermal barriers holding the heat in.

Alan
 

bohem

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Re: Do black barrels cool faster?



<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Jthoss0837</div><div class="ubbcode-body">Use a gold barrel. (Joking)
Reflects the most radiant heat.
Ferrari uses a gold type of paint to coat the engine bay, to reduce heating </div></div>

It's gold foil glued to fiberglass weave as a heat insulator to both reflect and insulate the engine bay heat from causing damage to the expensive CF bodywork/aero pieces.
 

sotexhill

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Re: Do black barrels cool faster?

Lots of vartiables here. Breeze/no breeze. Airspace around the barrel in the stock or no airspace. Daytime sunlight/night time dark. Wish there was a good clear answer. Barrels get hot here in South Texas.
 

c1steve

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Re: Do black barrels cool faster?

Well I came to the right place for the answers. I find black body radiation quite fascinating, and now I know more than ever.

I am in the north, colder more than hotter. Sounds like black will cool down 20% faster than green. I will keep these replies somewhere safe on my computer, proprietary information, if you must know.
 

sobrbiker883

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Re: Do black barrels cool faster?

Anyone have an excedrin?



Seriously, there is a lot of really surprising info, but I really need to ask myself (since I've got a barreled action to paint):

Can I shoot the difference??
 

ARCOREY

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Re: Do black barrels cool faster?

I would think the composition of the steel would play more into heat dissapation than color. I have an SPS varmint that would cook bacon on the black barrel when unfired in the sun. It was blued with a matte finish. I had it Duracoated coyote brown, and it still is a heat magnet. I also have a stainless bull barreled AR that is roughly the same diameter barrel that is Kryloned black and heat is negligable in comparison. I've also noticed the aluminum free float tube which is also black, seems to cool the quickest of all of the parts. Nothing scientific here, just some observations I've noticed.
 

CanPopper

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Re: Do black barrels cool faster?

My black color ABS carbon fiber wrapped barrel cools the fastest by far.
 

bohem

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Re: Do black barrels cool faster?

ARCOREY-

I'm not saying that your observations are wrong, but the governing behavior is not correct. For radiation properties the surface finish and color are far more important than the composition for similar materials. Chrome Moly and SS have similar behavior, in fact, AL, Steel and SS all have very close emissivity constants if they're given similar surface coatings/colors/finishes. Shiny polished surfaces are below 0.25, dark colored coatings are above 0.75, where each falls in there is dependent upon the material, but you can see that an uncoated, shiny piece of AL is about 1/3rd-1/4th (or worse) the emissivity coefficient that a piece of dark coated SS would have.

Remember the whole question is barrel cooling though, and barrel cooling is a heat transfer problem composed of 3 pieces, of which the radiation is just a contributor.



<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: CanPopper</div><div class="ubbcode-body">My black color ABS carbon fiber wrapped barrel cools the fastest by far. </div></div>

Cheater
wink.gif
 
G

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Re: Do black barrels cool faster?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: c1steve</div><div class="ubbcode-body">Has anyone, inc. the military, done tests on barrel colors? I would expect that a thin black coating would cool a hot barrel slightly faster, perhaps 10-20%, but that is just a guess.

I know there is a formula, which revolves around Black Body Radiation. However the formula uses degrees Kelvin (-273 C) it looks like one would need a barrel to be several hundred degrees hotter than ambient temp to have much of an effect.

Just wondering if there is cooling disadvantage with camo or green barrels as compared to black. Someone must have tested this, would be great to know before ordering my first GAP. </div></div>

Search, Briggs & Stratton, or Cummins engine company, everything you wanted to know about applied outside coatings, to reduce heat from on going internal heat
 

acehigh

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Re: Do black barrels cool faster?

I think Continental or Lycombing or someone in the aircraft engine business did a similar testing regimen. They were known for their black engines. Now that I think about it I believe it was a nationally known re manufacturer of A/C engines. Their tests indicated, in their particular application, it(black paint) had little effect.

The results mentioned above are from memory so I might be mistaken. Maybe a google search would find more info if one was so inclined.

JH
 

drmarc

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Re: Do black barrels cool faster?

I remember some years ago reading the results of a test that covered this subject and black did radiate heat the best but was offset if in the sunlight by it's absorptive properties.

Like c1steve said above, a heavily bead blasted barrel will dissipate heat the best when talking barrels.

It would be nice if they could use the same technique as dental implant manufacturers to increase surface area with microscopic spheres, I suspect it could increase heat dissipation probably 4 or 5 fold over standard bead blasting.
 

c1steve

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Re: Do black barrels cool faster?

It might increase surface area, but as soon as the barrel was coated, the coating would smooth over micro roughness such as that.

One thing that people might overlook is that stainless steel expands far greater/faster than carbon steel. I suspect it moves bout 6 to 8 times as much as carbon steel at 600 degrees. I do not have figures on this, just going off of observations when cutting stainless with a plasma torch.

Therefore a stainless barrel, when hot, might expand on the outside slightly. The inside would probably also grow larger, not sure how much. The bullet would probably always provide a seal to the barrel, though.
 

bohem

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Re: Do black barrels cool faster?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: c1steve</div><div class="ubbcode-body">It might increase surface area, but as soon as the barrel was coated, the coating would smooth over micro roughness such as that.

One thing that people might overlook is that stainless steel expands far greater/faster than carbon steel. I suspect it moves bout 6 to 8 times as much as carbon steel at 600 degrees. I do not have figures on this, just going off of observations when cutting stainless with a plasma torch.

Therefore a stainless barrel, when hot, might expand on the outside slightly. The inside would probably also grow larger, not sure how much. The bullet would probably always provide a seal to the barrel, though. </div></div>

All else being equal, CTE of common barrel steels:

SS 416R = ~ 6.2 uin/in-F (temperature dependent so that's the appx value)

Reference SS

CM 4340 = ~ 7.1 uin/in-F

Reference 4340 here

I didn't find the CTE for 4130 easily but 4340 is close enough to give you the idea.

Regarding the barrel OD change
 

romer522

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Re: Do black barrels cool faster?

Strange topic for my first post but here goes.

There are three ways energy can move in "heat" form, conduction, convection, and radiation.

When looking at the cooling of firearms as a single body we can pretty much eliminate the conduction piece of the puzzle.

Convection is the direct transfer of heat to the air which then will form air currents bringing more cool air into contact with the barrel, this is where fluting etc will come into play.

Radiation is the energy radiated in the electromagnetic spectrum, everything that has mass is constantly emitting and absorbing this form of energy. Emissivity is a characteristic of a surface which relates to how well that surface will emit, absorb, and reflect energy. The maximum possible emissivity is 1 which as has been previously stated is known as a black body, a perfect emitter and absorber of thermal radiation where none of the energy is reflect off the surface.

Anyways long story short, color really has nothing to do with it. Bare metals have extremely low emissivities and are therefor extremely inefficient at radiating heat energy. This is why coffee pots etc have polished stainless finishes and why many insulated mugs have a back to back stainless finishes separated by a vacuum. A polished stainless steel rifle barrel is going to be the exact same, unable to release its heat in the form of radiation. A bead or oxide blasted barrel is going to be marginally better, but still low on the overall scale.

Surface condition is always going to play a bigger part in emissivity than color, anything shiny, smooth or glossy is going to have a lower emissivity and therefor not radiate heat as well.

Good news for gun owners is that pretty much every coating out there with a matte finish is going to have an emissivity above .9 (same goes for matte interior house paints including white). Blued finishes are going to be dependent on whether it was polished or blasted etc. I've personally verified parked metal, ionbonded metal, and a couple ceramic coatings with an infrared camera to verify they are .9 or higher. Its hard to accurately measure stainless steel barrels because of how reflective they are. As stated above, avoid anything glossy because for the most part if it reflects visible light it also reflects thermal radiation. (thermal reflectivity and emissivity are inversely proportional)

Now, the part of color that does matter, which was hinted at above, is that black stuff gets hot in the sun. The majority of our suns energy is emitted in the visible spectrum not in the infrared, so black surfaces will absorb that energy much better than lighter colored surfaces. So if your concerned with the solar heating of your firearms finding a lighter shade finish that still has a very high emissivity will yield the coolest barrel.

Fun fact is that your barrel is going to cool substantially faster on a clear day compared to an overcast day. This is because on a clear day your barrel is literally radiating its energy into space, and space is COLD so its not emitting much back into your barrel, the atmosphere does provide a small insulating effect but its very minimal. When its overcast you now have a substantial layer of thermal radiation insulation between your barrel and space.

This has always been an interesting topic for me as I have never understood the use of bare stainless steel rifle barrels in any situation where heat buildup is a concern. I am still hoping to do a direct semi-scientific comparison one of these days when I get some spare time and get a manufacturer to donate me some chopped-off pieces of stainless barrel blanks.

For anyone more interested just google "emissivity of materials" and look for some lists. You'll notice that far and away most materials in our world are high emissivity and that bare metals comprise the vast majority of low emissivity materials.
 

c1steve

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Re: Do black barrels cool faster?

I am now going to order an OD barrel, not black. Thanks for all the help on such a fascinating topic.
 

TAC

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Re: Do black barrels cool faster?

Damn romer, Schooling on your first post... Thanks! I always like to see a scientific mind applied to what it is we do and you presented it very well. My thing is TXRF but material science is a close second.


Cheers (raising my G&T).

TAC