Myth of brass carrying away heat from a chamber
- Thread starter -v-
- Start date
- Status
Full auto MG's can't take continuous fire, they've been water cooled, or needed to be cycled in bursts to keep the heat load from exceeding the working temperature of the barrel.
That alone should put to rest the myth of the case carrying off heat - but no, it keeps popping up. Well, add open bolt actions, which are used to allow air into the chamber and barrel to cool it. Submachine guns fire from an open bolt, and Colt even has a combo action that fires closed bolt for accuracy, but when it warms up, switches to open bolt without the operators intervention. I haven't seen or trained on a MG in my career that didn't. MG's don't load rounds into hot chambers to cook off, they stop in the open bolt condition. Moot point what a cartridge will do - it's accepted it WILL cook off.
All that was invented and made reality with brass cases supposedly carrying off all that heat - which they simply can't do all that much. If the M4 video of full auto firing until the barrel explodes doesn't tell you that, or stories of soldiers pouring water on MG barrels in combat, then the physics in numbers won't either.
As for "caseless," the LSAT polymer cased round can fire full 100 round belts, there's plenty of video released by .Gov to show it, and tests are scheduled for Battalion level exercises. That's 400 weapons being used - not just a lab sample knocked up out of one off parts.
Look at the evidence right out there and come to your own conclusions.
That alone should put to rest the myth of the case carrying off heat - but no, it keeps popping up. Well, add open bolt actions, which are used to allow air into the chamber and barrel to cool it. Submachine guns fire from an open bolt, and Colt even has a combo action that fires closed bolt for accuracy, but when it warms up, switches to open bolt without the operators intervention. I haven't seen or trained on a MG in my career that didn't. MG's don't load rounds into hot chambers to cook off, they stop in the open bolt condition. Moot point what a cartridge will do - it's accepted it WILL cook off.
All that was invented and made reality with brass cases supposedly carrying off all that heat - which they simply can't do all that much. If the M4 video of full auto firing until the barrel explodes doesn't tell you that, or stories of soldiers pouring water on MG barrels in combat, then the physics in numbers won't either.
As for "caseless," the LSAT polymer cased round can fire full 100 round belts, there's plenty of video released by .Gov to show it, and tests are scheduled for Battalion level exercises. That's 400 weapons being used - not just a lab sample knocked up out of one off parts.
Look at the evidence right out there and come to your own conclusions.
fletcher
Member
Just because a MG overheats does not mean the case is not carrying away heat. It isn't taking away all of the heat, but it most certainly is taking away some.
Husker_Fan
Member
-v-,
You sound like an engineering student. Have you tried thinking through what a mass and energy balance for the chamber would look like?
If you did one, you'd realize your flaw.
You sound like an engineering student. Have you tried thinking through what a mass and energy balance for the chamber would look like?
If you did one, you'd realize your flaw.
Husker: Actually, no. Life sciences and now medicine is more my field right now. If you want to talk about poorly understood systems with multiple variables that are all in a state of dynamic flux, and where we don't really even know what half of them are or what their state is, well that's right now more of my thing. In that regard I really see engineering as a lot simpler, we know everything that is going on and can calculate it. Not so when someone is sick. Also, I appreciate the complement, as I've only had 2 introductory physics classes...8 years ago.
Now, I will interject that there is some issues with the terminologies thrown around. What I was more concerned was with "ejecting hot brass cools the chamber" as in reduces the temperature of the chamber. What this turned into is calculating total thermal energy or heat of the chamber. Its no debate that a hot casing carries off thermal energy - if it didn't it wouldn't be hot. My initial thrust was at the notion that ejecting a hot casing reduces the temperature of the chamber - which is just utter bollocks.
Now, I will interject that there is some issues with the terminologies thrown around. What I was more concerned was with "ejecting hot brass cools the chamber" as in reduces the temperature of the chamber. What this turned into is calculating total thermal energy or heat of the chamber. Its no debate that a hot casing carries off thermal energy - if it didn't it wouldn't be hot. My initial thrust was at the notion that ejecting a hot casing reduces the temperature of the chamber - which is just utter bollocks.
The inverse is obviously false... (Ejecting a case does not increase the temperature.)
So ejecting the case either has a net effect of nothing, or of reducing the temperature of the chamber. I suspect that removing the case increases the rate of heat transfer [ dQ/dt ] from the hot chamber to the environment. (Specifically, by opening the breech and better allowing air to remove heat from the inner chamber walls.)
The chamber may not become cooler the instant that the case is ejected, but it should cool faster between shots, and thus remain cooler over continued cycles. I'm not sure what else you are looking for here -v-.
If the brass stays in the gun one would not be able to introduce a new cartridge which would allow the gun to cool. So even then the brass case cools the gun.
351 WINCHESTER
Member
Ever wonder why brass from a semi or full auto is hot, but from anything else is not? I figure it's from friction.
MachIVshooter
Member
You figure wrong.
Don't Feed The Engineers. I like that.
Anyone care to actually put how many BTUS's a case is being carried off?
As for cooling anything, the point is moot - not enough to effectively prevent things from overheating, cooking off a chambered round, or distorting the barrel. The question is academic in the sense that engineers have addressed it - not enough can be done. Firing one can't be done in a continuous manner or it will be damaged.
However much heat might be carried off, it's ineffective, and changing to polymer cases hasn't apparently stopped the LSAT at all.
Define some concepts with real world measurable terms, oh ye engineers - what is the temperature of the 99th case when ejected, how many BTU's is it carrying away, what proportion of the total propellant heat discharge does that represent?
Makes little difference - 9mm or .50BMG, the case isn't a major factor, despite the much larger amount of heat carried off by one vs another. Continuous fire elevates chamber and barrel temps anyway, and the now standard practice of firing in bursts and using an open bolt is still the solution.
Anyone care to actually put how many BTUS's a case is being carried off?
As for cooling anything, the point is moot - not enough to effectively prevent things from overheating, cooking off a chambered round, or distorting the barrel. The question is academic in the sense that engineers have addressed it - not enough can be done. Firing one can't be done in a continuous manner or it will be damaged.
However much heat might be carried off, it's ineffective, and changing to polymer cases hasn't apparently stopped the LSAT at all.
Define some concepts with real world measurable terms, oh ye engineers - what is the temperature of the 99th case when ejected, how many BTU's is it carrying away, what proportion of the total propellant heat discharge does that represent?
Makes little difference - 9mm or .50BMG, the case isn't a major factor, despite the much larger amount of heat carried off by one vs another. Continuous fire elevates chamber and barrel temps anyway, and the now standard practice of firing in bursts and using an open bolt is still the solution.
Andrew Wyatt
Member
Tirod: The Lsat uses plastic cases which are an insulator, and you're not defining continuous fire.
in equal length bursts,with equal propellant charges, a cased gun will be cooler than a caseless gun, because the cases absorb heat.
in equal length bursts,with equal propellant charges, a cased gun will be cooler than a caseless gun, because the cases absorb heat.
MachIVshooter
Member
http://www.youtube.com/watch?v=8CRXH-muly0
That's going to change whenever a variable changes. There is no simple figure.
What propellant?
How Much Charge?
How long is the bullet in the barrel?
What's the lock time?
How thick are the case walls?
What's the R-factor of the chamber wall?
What's the R-Factor of the case?
Those, and about 50 more variables need to be known for your calculation.
For example, one cannot say a 5.56mm cartridge case carries away X number of BTUs because, all else being equal, a casing out of an M4 is going to retain more heat than one out of an M16; The bullet is in the barrel for a shorter time (less duration that the case is pressed tightly against the chamber walls) and because it is in the chamber for a shorter time.
HoosierQ
Member
I don't think it is so much a case of taking heat away in that active sense. But rather a case where there are two places the heat is stored after a round is fired the case and the barrel (and a third if we count the air, the gas the exits with the bullet). So on this subject a hot part is discarded and is left to be cooled the air around it and the ground (or rather to heat up the air and the ground) and thus a measure of heat does not then have to dissipate from the gun itself.
Right?
Right?
BCRider
Member
Yep, I'm afraid that MachIVshooter got you on that one. The brass from the "anything else" is only cool becuase it was in the chamber long enough to cool off by passing the heat to the barrel and reciever.
It may not seem like a big deal in a bolt action rifle that sees at most around 25 to 30 rounds a minute even in battle conditions. But consider that machine guns, despite tossing away much of the heat with the cases, require barrel changes or some form of cooling system when shooting large amounts over a long time.
You're only seeing cool cases from a bolt rifle because it simply isn't firing often enough to get overly warm despite keeping the brass inside long enough to fully transfer the heat.
BCRider
Member
v said:
You've GOT it....
As I suggested earlier it was a bad choice of phrasing. The ejected brass carries away some amount of heat that avoids the chamber, barrel and reciever, getting hot as fast as it would otherwise. But it is very poor use of language to say that it reduces the temperature of the gun parts.
Sadly our use of the spoken and written word is one of the victims of our modern permissive society that fails to put a price on how we express ourselves. This is simply a minor example of what we have lost. You may as well get used to it and learn to read between the lines like the rest of us....
I wish someone would get a bolt action rifle, fire one shot eject the shell, than use a heat gun to measure the temperature of the bolt body, handle, and barrel at three points. Than take the same rifle fire a shot and than repeat the same measurements but without ejecting the spent casing. Than someone will actual have some data to back up their claim.
Art Eatman
Moderator In Memoriam
I'm just glad that my Thermo prof can't see this thread...
- Status
