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Thread: Heat damaging a barrel?

  1. #1

    Heat damaging a barrel?

    In a follow on from the barrel life thread I have some questions. I've often seen it said that you should let barrels cool between strings of shots to prevent excessive barrel wear but after some consideration I'm given to suspect that there can't be any foundation for this claim.

    What is the mechanism by which shooting a warm barrel damages the bore? If you consider that the barrel is normally at 20 degress, or about 293K, under ambient conditions (OK, not anywhere close this winter/spring but I need to start somewhere) and that it needs to approach 80 degress or about 350K to be too hot to hold, which is the measure of "too hot" that seems most commonly used on the forums I frequent, then how does this cause any change in state of the steel barrel? In turn how does this change in the state of the steel cause the barrel to wear out?

    As an engineer, though with no knowledge of this specific area at all, I fully believe that barrels wear out and that different cartridges, and different loads in a given cartridge, can result in different rates of wear. What I will not believe, until someone can describe the exact mechanism, is that shooting a warm barrel, at a few tens of K above ambient, can have any significant impact upon the life of a barrel which is regularly subjected to extreme high pressure events with a flame front temperature measured in thousands of K. I just can not see any mechanism by which starting say 60K higher than ambient can be a major or significant factor in an event which might occur at 6,000K above ambient.

    As an aside to this despite many questions on various forums I have yet to find the potential life of my Blaser barrel as I can't find anyone who has worn one out. This can only be a good thing.

  2. #2
    All I know is, when we used GMPG's in the SF role we would fire them until the barrels glowed red hot and then changed em an threw them on the ground to cool. Didn't seem to affect the shot placement but I do accept true accuracy was not what it was about, e.g. beaten zones etc.

  3. #3
    I also remember when the empire was fighting the zulu's at Rorkes drift [not personally before you p#s* takers start ] it is often spoken off that the Martini rifles would fire when the breach was closed as the rifles were so hot they set the round off before the trigger wasn't pulled.

  4. #4
    As far as I can see, the heat of the shot is retained by the barrel. As that heat builds up the steel becomes more malleable and the leade of the rifling takes a batttering every time a bullet is swaged into it. If long strings of shots are fired without giving the barrel a chance to cool the leade of the rifling becomes "blunted". The two best examples I can think of to show this are both military related. As mentioned above, in a GPMG barrel, if burst lengths are not observed starts to show visible erosion of the rifling after as little as 400 rounds (admittedly, heat build up is far more rapid). The other is that on the introduction of cordite cartridges to British service the increased heat was said to "wash" out the rifling in Metford barrels. Although both examples are extreme cases, the same forces are in play when a rifle barrel is fired hot.
    That's my take on it.
    You can't say muntjac without saying, Mmmmmm.

  5. #5
    A barrel heats up the carbon that is deposited from the burnt powder, every time a round is discharged that carbon heats up to such a degree that it etches the steel it is deposited on if it is not left to cool. The more carbon the more intense the heat, the more it degrades.
    Hence the need for cleaning..


  6. #6
    For me the problem with both of these ideas is that steel transfers heat very rapidly. If you were to hold a metal spoon, say, by the handle and suspend the other end of it in a flame then I suspect you would not hold it for very long. So, the idea that general inter-shot conditions within the bore are, somehow, at a significantly higher temperature than the exterior over a string of ten shots doesn't make much sense to me. Griff's suggestion that carbon in the bore somehow holds the heat is, at first, a good one but then I am given to consider that there isn't enough carbon in the bore to prevent the passage of the bullet and this rather small amount of carbon isn't going to hold a lot of heat, especially when this heat is being "sucked" away by the steel barrel. Also neither idea, although good, gives me any reason to believe there has been any sort of physical or phase change in the barrel steel which would result in the next shot causing more damage to the steel than the previous one. So, if heating the barrel from 290K to 350K does indeed cause increased wear then I'm still inclined to suspect that we haven't found the reason.

    So, my position remains that while the steel of the barrel may warm up a little I can still see no evidence that it ever gets to a temperature sufficient to change the properties of barrel steel and so I can find no evidence that the sort of sustained strings that us stalkers might shoot (as opposed to the machine gun example) can promote extra wear to a barrel over and above the wear caused by shooting it under normal circumstances.

    I can see that a machine gun might be used to heat the barrel steel to a temperature where the properties of the steel might change and that under these circumstances wear rates will increase but, as is illustrated by the fact the barrel becomes red hot, this heating is rather more than the few tens of degrees we see with a stalking rifle. I can also understand that if a "hotter burning" powder is used then this will increase shot to shot damage through the normal processes of high pressure and extremely high temperature during the progression of each shot but I would argue that this is not the same effect as having this errosion process accelerated because the barrel had reached a temperature of about 350K, was no longer comfortable to hold, and so the metal had changed in some fundamental way.

    So, while I'm probably wrong I remain unconvinced that I have seen any explaination that barrel steel behaves differently at 350K compared to how it behaves at 290K.

  7. #7
    The hotter a metal becomes the softer it becomes but also there is the issue of expansion so once a rifle gets 'hot' and accuracy is marginally reduced because the 'fit' is getting a bit pants. Soft metal wears quicker than hard metal obviously but only a tiny bit at realistic operating temperatures.

    The most serious risk to accelerating a barrels wear is when they're left with small particles in them that key onto the bore wall before shots are taken. Pulling through before and after rifle use greatly reduces such risk but it's easy to mark a barrel if it's shot through without taking care to check it first. Once it's marked the wear around the mark gets worse while the rest of the bore might remain fine for quite a while.
    Last edited by Paul at Fechan; 08-05-2010 at 18:17.

  8. #8
    Remember looking at material tables for chemical resistance, huge difference between RT (20 C) and even just 50 C, corrosion went up exponentially with increase in heat.
    Steel also looses mechanical strength fairly soon when warming up.
    Big problem with stainless is it has a very low heat transfer compared to mild steel.
    Meaning the rifling might be still very hot when the next shot is being fired.

    it all adds up


  9. #9
    when steel is heated it expands even with tiny temp. changes so i guess the tiny change could affect the how the bullet travels down the barrel, maybe not engaging the rifling so well and scratching of more steel than in cold barrel. not sure tho just a though.


  10. #10
    Your forgetting the extreme pressures involved and it's the surface temp for the top few thou that matters. When that bullet slams into the hot steel it tears out tiny chunks of steel and drags them up the bore the exposed surface is then ravaged by super heated gases and blasted by the unburnt kernals of powder hence the wear to the throat of your barrel. The hotter you allow it to get the faster the errosion caused................................. Simples

    It's this heating and cooling that cracks or crazes the surface of the steel and once that occurs errosion increases as the bullet grabs at it more and so tears out more each time. The higher the pressures and the higher the velocity the quicker the wear and errosion.

    Now if you want to be shocked or frightened then get someone to poke a bore score down the barrel, after you've cleaned the fouling out of course so you can actually see the surface of the steel, and prepare to be shocked. I know I was.

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