Myth of brass carrying away heat from a chamber

[-v-]

I am really curious where did this myth that ejecting brass cools the chamber come from? For this to be true, it would have to break the second law of thermodynamics.

Which says that "heat flows from things that are hot to things that are cold, and that if two things are equally hot, there is no net heat flow" (paraphrased) unless their is an insulator in the way.

So, coming back to the myth of brass carrying away heat. You put a round in the chamber, and it has powder in it. The powder is ignited and it turns into gas + HEAT. This heat heats the brass, which being an excellent conductor of heat, heats the chamber. So now, the brass is hot, and the chamber is equally as hot as the brass. Brass is now ejected, but the brass has already transferred heat the chamber, so the chamber remains at that new temperature.

For the brass to cool the chamber, the heat would have to go into the brass, and make it even hotter then the chamber, so something cold would have to get even colder and something hot even hotter.

An other analogy is put a spoon in a pot of water, boil the water, now remove the spoon from the water. Did the water get cooler?

 

[rcmodel]

Myth??
Brass carries away heat from the gun is a myth???

You spent way to much time book learn'n, and not enough time laying in the mud getting hot machingun brass sprayed down your starched Army Green shirt collar son!

Nobody said it cools the gun.
They said it keeps the gun from getting hotter then it already gets if there was no hot brass being ejected still holding a lot of heat every round.

put a spoon in a pot of water, boil the water, now remove the spoon from the water. Did the water get cooler?

No, not if you boil the spoon with the water.
But it would have if you had put a cold spoon in the boiling water, then took it out, dipped it in cold water, then put it back in the hot water, cooled it, and did it again and again.

rc

 

[SleazyRider]

Perhaps it's the incoming round to which they're referring, because it seems to me that the incoming round enters the chamber at ambient magazine temperature and instantly absorbs heat from the chamber, cooling it somewhat.

 

[taliv]

Is the brass hot? Is it still hot when it leaves the chamber? I guess it carried heat away.

I think what you're missing is the comparison to caseless where all the heat from the powder burning would transfer into the barrel steel instead of a substantial portion staying in the brass

 

[-v-]

Myth??
Brass carries away heat from the gun is a myth???

You spent way to much time book learn'n, and not enough time getting hot machingun brass sprayed down your starched Army Green shirt collar son!

The definition of a physical law is everything in the god mighty universe obeys it. You, your machine gun, your second cousin three states over, and every other particle in the universe.

Nobody said it cools the gun.
They said it keeps the gun from getting hotter then it already gets if there was no hot brass being ejected still holding a lot of heat every round.

Lots of that heat goes out the barrel as the hot gas. Some of it turns to work to move the bullet down the chamber, and some is lost as friction between copper and steel. If you are referring to most MGs firing from an open bolt, that has more to do with letting convection cool the barrel then a flimsy piece of metal. Again the spoon in the pot of boiling water analogy. If what you say is true, then having a line of spoons dip into the hot water constantly should keep it from boiling.

For that matter, does a bolt action rifle get hotter from not ejecting a cartridge from it? And does it suddenly get cooler or not get any hotter then X by cycling a cartridge through it? Sure, when the barrel is the same temperature as the hot gas generated from the cartridge, it wont get cooler, but then thats not really a big surprise.

SleazyRider:
I could see the incoming round cooling the chamber, but seeing as brass has a pretty crappy heat capacity, it'd be very little cooling at all.

taliv:
Brass has a lower specific heat (amount of energy needed to raise its temperature 1 degree) then steel. Considering the mass of a chamber vs the mass of a brass cartridge the, amount of heat energy it would absorb and remove would not be that great. While I agree with you that caseless would transfer all of its heat energy to chamber vs brass, I do disagree with you that it would be that much more in raw joule amounts of heat - eg 5-10% (or something similar) more due to the vast disparity of mass of the chamber versus that of most rifle cartridges.

 

[CAR-AR]

When I shoot my PSS the ejected brass is cool to the touch.

748
165 gr bullet

 

[paintballdude902]

opening the chamber helps cool the barrel, especially if you leave it open. it allows cooler air to flow through

 

[taliv]

So there's arguing about whether or not the laws of physics allow brass to carry away heat from the chamber and there's arguing about how much and whether it's material.

It sounds like we've progressed from the former to the latter. I won't debate that but I will say you're neglecting the time the brass stays in the chamber. Regardless of their relative densities their takes time for the heat to transfer and 800-1200 rnds per minute doesn't leave much time for the heat to leave the brass and move into the chamber.

 

[BCRider]

"Cools the gun" is an unfortunate turn of phrase. Certainly throwing the brass away does not "cool" the gun. Same as removing the spoon from the boiling boiling water does not cool the water or cup. It is far more accurate to say that ejecting the brass quickly such as in a semi auto prevents the gun getting as hot as it COULD get.

All metals have a coefficient of thermal conductivity. It's a measure of how fast heat changes take place at various points along the path through the metal. It's why heat sinks for semi conductors handling a lot of power are heavy and thick directly around the contact point and get thinner the further away the base and fins are from the heat source.

What's this have to do with a rifle? On a semi auto rifle the ejection takes place very quickly after the ignition of the charge. The very thin walls of the brass casing do not have time to pass all the heat from the inside to the outside before they are thrown clear. So they carry away heat that had not finished traveling through the wall and into the chamber. In this way they aid in keeping the chamber and surrounding area of the gun somewhat cooler than they would be if the casing was left in place and the heat was allowed to equalize between the casing and the chamber.

Of course bolt action rifles don't enjoy this ability to toss away excess heat while it's still in the walls of the casing. Even the fastest bolt operator is many orders of magnitude slower than the cycle of a semi auto rifle. So in the case of bolt rifles it becomes very much similar to your spoon removed from a cup of hot water idea.

It raises a point about the need to deal with this heat with the new caseless ammo which is being developed and experimented with. In that case there's no brass case to carry away the heat before it can travel out and into the chamber walls.

 

[Andrew Wyatt]

_V_: you are incorrect in several respects.

1. you are assuming that the heat is totally propagated faster than the time it takes for the case to be ejected from the gun.

2. you don't have any data to back up your hypothesis.

 

[bigfatdave]

I think you're ocnflating the concepts of "heat" and "temperature"
Hot brass exiting the gun obviously takes a given amount of mass at a goven temp. away with it, that's a type of heat, in this case ... a measurable quantity of energy stored in temperature and/or phase changes within matter.
If the ammunition was caseless or otherwise free of having an ejecting casing, you would have more heat and more rapidly rising temp. in the system.

This might be easier to describe with diagrams and examples ... I don't want to pull an argument from authority, but I've been playing with thermodynamics professionally for quite a few years and I still have to explain the difference between heat and temperature to engineers and executive types, it takes more than plain text.
Start by reading and understanding the way heat is measured ... start with the BTU, which is adorably antiquated but easy to envision:

British thermal unit (symbol Btu or sometimes BTU) is a traditional unit of energy.
It is approximately the amount of energy needed to heat 1 pound (0.454 kg) of water, or about 0.1198 US gallons, from 39°F to 40°F

Now compare that to simple temp. unit, the "degree Fahrenheit" which is just a measure of the ambient energy, and is completely divorced from concepts like "how much mass at that temp." or "how much hotter/colder than the surrounding material"

This is already too long, for more you'll have to pay me or something! (I accept cash, precious metal, ammunition, or fuel)

 

[WNTFW]

Can someone explain to me why I should care?

I am more worried about almost every other aspect of shooting. Wind velocity and direction, range, my hold and the like. I just know brass is hot when it ejects from the guy on my right. So avoiding it is more useful than does it change the chamber temp by 5 degrees.

So long as the heat isn't transferring to my skin I'm good.

 

[-v-]

BCRider: Thanks, that is something I didn't take into account, and it really ties it all together quite well.

Bigfatdave: That's a bit amusing that you trimmed out the metric stuff, I actually tend to approach most of this stuff in the metric sense, versus engineering units. (have a life sciences background) But, yes, I am familiar that specific heat is amount of energy needed to raise 1 gram of a substance by 1 degree Celsius.

 

[FlyinBryan]

so is it a myth still? or does ejecting hot brass dispatch heat again? (all mine comes out hot, some hotter than others, but all hotter than the temperature of the surrounding environment)

 

[Robert101]

Not sure if I get what this is all about. The amount of heat absorbed by so little brass [in weight] compared to the weight of a steel barrel is very minimal. Its like saying, remove a couple of drops of water from a full bucket......... do you still have a full bucket of water?

 

[MachIVshooter]

So, coming back to the myth of brass carrying away heat. You put a round in the chamber, and it has powder in it. The powder is ignited and it turns into gas + HEAT. This heat heats the brass, which being an excellent conductor of heat, heats the chamber. So now, the brass is hot, and the chamber is equally as hot as the brass. Brass is now ejected, but the brass has already transferred heat the chamber, so the chamber remains at that new temperature.

First problem (as pointed out by others) is the assumption that heat transfer is instantaneous. In the case of autoloading firearms, the brass is extracted from the chamber long before the heat transfer reaches equilibrium.

The second flaw is in the statement "so the chamber remains at that new temperature". This assumes that the chamber does not dissipate heat into atmosphere, retaining all the thermal energy that was generated within.

An other analogy is put a spoon in a pot of water, boil the water, now remove the spoon from the water. Did the water get cooler?

Of course not, if it was heated with the water. But take the same spoon at room temperature and put it in the water long enough for it to heat up to the same temperature as the water. Assuming for the moment there are no other variables (continued application of heat to the pot, cooling of the water as it's heat dissipates into atmoshpere), that spoon will have slightly lowered the temperature of the water; It became a (rather pitiful) heat sink.

As already covered, the concept of the casing carrying heat away is in comparison to caseless ammunition. Caseless weapong have problems with "cook off" because 1) the hottest point during the ignition cycle happens in the chamber, and there is no case to insulate the chamber from the burn and 2) there is no case to insulate the fresh round's propellant from the hot chamber.

No one is claiming that the brass actually removes heat from the chamber; It simply prevents as much of the heat from being transferred to the chamber during ignition. Yes, if the case is left in the chamber for a long enough duration, the chamber and case will be the same temperature. But extraction happens in milliseconds, and it takes a lot longer than that for the heat to do the kind of transfer you're envisioning.

If you ever have a chance to play with a cutting torch, you'll better understand the reality of heat transfer. I can blow a mechanically welded hardened steel bolt out of cast iron hole without even destroying the threads in that hole. Of course the cast piece will be very hot, but much cooler than the piece I had the flame focused on-even though they were in direct contact, and for a lot longer than the cartridge case stays in the chamber of an autoloading weapon.

Heat transfer is not a simple subject, and there's a lot more to it than a layman understanding of basic thermodynamics.

 

[JohnKSa]

This heat heats the brass, which being an excellent conductor of heat, heats the chamber. So now, the brass is hot, and the chamber is equally as hot as the brass.

In addition to BCRider's points, there's another.

Even if we assume that the case isn't ejected until equilibrium is reached, the overall temperature of the brass AND the chamber will be less because with the brass present, the heat is distributed over more mass.

So you get at two effects that reduce the amount of heat left in the chamber. The brass is probably ejected before equilibrium is reached, and in addition, because the brass adds to the mass that must be heated, the overall effect is a lower temperature for both the brass and the chamber.

 

[maskedman504]

So this is what trying to paddle upstream is like....

 

[tundraotto]

I can give a guess that the energy by the fired cartridge has put more heat in the barrel/action that the tiny weight brass case has taken away.....what is this? some kinda joke?

 

[bigfatdave]

Even if we assume that the case isn't ejected until equilibrium is reached, the [strike]overall temperature[/strike] of the brass AND the chamber will be less because with the brass present, the heat is distributed over more mass.

stored heat, not "overall temp."
And of the gun in general, because obviously the firing-imparted heat remains in the brass as it cools to ambient.

 

[hogshead]

Shoot automatic high powered rifle ,retrieve brass, stick your tongue to it . Then tell us if it is hot. If so then brass removed possible heat. Please video this for us.

 

[Owen Sparks]

People are always concerned about their barrel getting hot and effecting accuracy. How much effect does heat have on the cartridge that goes from the outdoor temprature to however hot the chamber is? That HAS to have some effect on point of impact. Suppose it is 40 degrees outside and a cartridge goes from a cold magazine to a hot chamber and stays there long enough to warm up to over 100 degrees? Will it hit exactly where that first cold round hit? I think this is the cause of the "lonely" first shot.

 

[Ragnar Danneskjold]

You spent way to much time book learn'n,

No such thing. "Book learn'n" is why we are using guns and not sharp rocks.

 

[FlyinBryan]

can spectators "bow out" ?

 

[Certaindeaf]

So the moral of the story is to blast some freon down the tube?