spent case temperature difference

S

stagpanther

New member
OK--this is probably one of the top dumb questions I've ever asked--but it really has me wondering.

Yesterday I was shooting my AR creedmoor build when an ejected shell bounced off a board and hit my hand--burning it cause it was so hot. Later while shooting my ruger american 270 I noticed the spent cases coming out of the chamber were cool to the touch even when the barrel was quite warm. I know the AR sends gas back into the receiver--but I still don't see how that can account for such a marked difference. Why is there such a big difference?
 
the 270 case was in the chamber longer and some of the heat was transferred to the action. the 223 case wasn't in the action long enough to transfer any of the heat
 
the 270 case was in the chamber longer and some of the heat was transferred to the action. the 223 case wasn't in the action long enough to transfer any of the heat
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Creedmoor case in the AR--but the 270 I ejected immediately. I still don't see how heat can conduct out that fast from the 270--which has more powder--than the creedmoor in an AR. : )
 
the 270 case was in the chamber longer and some of the heat was transferred to the action. the 223 case wasn't in the action long enough to transfer any of the heat
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That has to be the answer. Last Saturday I was shooting my Saiga 223 and had a spent case bounce off the deflection screen next to the bench and hit my arm, it was very hot. But shooting my Remington 700 in a belted magnum I could eject the spent case with the bolt and grab the empty as it came out, it was cool enough to handle. A belted magnum surly burns hotter than a .223.

I've never given it much thought, but that extra 1 or 2 seconds that it takes to manually open the action of a bolt gun must be enough to cool the empty case down.
 
I've never given it much thought, but that extra 1 or 2 seconds that it takes to manually open the action of a bolt gun must be enough to cool the empty case down.
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Trouble is--the spent case from the AR stays hot for many seconds even out of the chamber--which tells me it retains heat well
 
Chances are more rounds were fired in the seconds before the one than burned you with the AR, making the chamber hotter to begin with, and gas under pressure was in the case longer too. It could also be that the vigorous extraction of the AR imparts some heat.
 
Chances are more rounds were fired in the seconds before the one than burned you with the AR, making the chamber hotter to begin with, and gas under pressure was in the case longer too. It could also be that the vigorous extraction of the AR imparts some heat.
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I could be wrong--but I'm pretty sure even the first round out of the AR ejects a hot case.
 
I played with this one time, stroking a bolt or lever as quickly as I could. The cases were always considerably less hot than one ejected from an automatic in a fraction of a second.

The heat carried away by immediate ejection of the brass from an auto is one reason caseless cartridge guns are so hard to build. No disposable heat sink.
 
I think to really do an experiment along these lines you need something along the lines of apples to apples and oranges to oranges. Thermodynamics and heat flow and transfer were not my forte but years ago I did some work with it. Heat and temperature are not quite the same thing. Heat flows and it always flows from hotter to colder unless both objects are the same temperature in which case it doesn't flow.

This past spring I did some experiments using my M1A chambered in .308 Winchester. The idea was to note the velocity of identical loads shooting the rifle with the gas port open and then closed. Grab a case on ejection with the gas on and I'll guarantee you catch that case and you will let go real quick. With the gas off I was waiting roughly five to ten seconds and manually cycling the rifle. Grabbing the cases as they came out of the chamber was not a problem, the cases were warm to the touch. Time between shots, gas on or gas off was about a minute. So the chamber is sinking the heat from the cases. The question is how fast the heat travels from the hot case to cooler chamber. How hot the case actually gets should be the result of a few variables like the powder used and charge size.

My real objective was the velocity with gas on and gas off which surprisingly didn't change to speak of. I also measured the cartridge base to datum on the shoulder which also did not change. I didn't get the differences I expected. Anyway, with identical ammunition the 5 to ten seconds in the chamber made a big difference in the case temperatures. That being just a side observation. How fast the heat flows? Not quite my forte. :)

Ron
 
It's not a good idea to do all this apples/oranges comparison. I just spent a while shooting my .243 bolt, and I was amazed to find that this current loading let the casings come out cool, barely warmer than the unfired rounds. Slow powder, starting load, and probably most of that charge was consumed at the very front of the chamber or down the barrel. Larger charges in various bolts created cartridges too hot to handle.

Looking at the pistol, I could see where that thin brass would be able to draw off a little heat while the rest of the rounds were held in place, and yes, I can see where a semiauto would throw them out before the heat sink of that massive cylinder could draw any of it off. OTOH, a 9 nn with a pretty much identical charge is crazy hot, where a larger capacity case stays cooler.

I have burned the poop out of myself collecting .223 brass for other people as they shot.
 
Everything fired out of a semi auto comes out hotter than the exact same thing fired out of a manual repeater, no matter how fast you try to work the action.

Do note that the powder gas, quite hot, is still present in the action (and the case) when it is ejected by a semi. There is still SOME pressure there, just not high enough to prevent extraction. I believe the short time in the chamber, AND the fact that the case is still in contact with hot gas when until it is ejected is what makes the difference.

No matter how fast you work the action of a manual repeater, it takes longer than the semi does, AND, by the time you get the action open manually, gas pressure (and heat source) is essentially gone.

Steel is a much more efficient heat conductor than air is. The very short (to us) difference in time between manual and semiauto extraction of the case may not seem like its long enough to bleed off that much heat, but obviously, it is.
 
My first thought was I believe gunpowder burns at different temperatures. One powder may burn cooler in a lower pressure chamber while another will burn at a higher temperature. Jes thinking out loud...

IIRC; brass immediately ejected from my 308 bolt guns is way hotter than brass outta my 30-06 Garand...
 
I do also see the probability of various rounds and various powders leaving widely varying amounts of retained heat in the cartridge, for what little worth that issue is worth.

We also should consider the fact that ejection probably starts before the case is fully returned to unfired size, and there is going to be some friction added to the existing heat from the powder charge.
 
the 270 case was in the chamber longer and some of the heat was transferred to the action. the 223 case wasn't in the action long enough to transfer any of the heat
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^^^This^^^

Do a test for yourself. Shut the gas system off on your AR, fire a round, eject it by hand, it will be cooler, much cooler.
 
Different powders, possibly different pressures, possibly different brass thickness, definitely different bore diameters.

But I think that powder burn rate is the biggest measurable difference I've dealt with in bolt action rifles. I've loaded up some 6.5x55 with really slow ball powder, WC872, and they were hot, but not painfully so when coming out of the rifle. Much cooler than milsurp 30-06 loaded with IMR4895. I'm sorry I don't have an apples to apples comparison to give.

Jimro
 
I load for an AR-10 308 and a Savage 308.

Same load same everything cept one stays in the chamber longer before being extracted.

The AR-10 brass will burn you, the Bolt action is nothing but comfortable to the touch.
 
"...has more powder..." Irrelevant. How hot a case feels depends on how long ago it was fired. A .270 will be sitting in the chamber longer than a Creedmoor in a semi. It's seconds in a bolt action, it's microseconds in a semi. A .22 LR empty will burn you out of a semi-auto. Out of a bolt action it won't.
It has nothing whatever to do with the burn temperature of the powder.
 
So far--the general opinions seem to be:
1. Depends on length of "stay" in chamber before extraction; and
2. conductivity of heat from case to chamber.

Interesting ;)
 
For women shooting, it depends on how many shirt buttons are opened
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FITASC: Fédération Internationale de Tir aux Armes Sportives de Chasse. http://www.fitasc.com/uk/home/view
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Now ya done it--you in a heap a trouble boy:p

Brings back memories of when I was training with police for a CCW permit and I was firing my 45 acp for target quals (which were easy enough probably a blind person would pass) I was wearing loose-fitting t-shirt and shorts--and a hot spent case hit my hat rim, went down my shirt and into the front of my pants. I did quite the dance. I bet they still laugh about that one to this day.:D
 
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