SS/CM barrel life.

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by 25 Otter, Jul 21, 2013.

  1. 25 Otter

    25 Otter Well-Known Member

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    Has anyone here found an advantage with either Stainless or Chrome Molly barrels with regard to erosion in the 6.5/284?
     
  2. westcliffe01

    westcliffe01 Well-Known Member

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    Speculation: The thermal conductivity of CM is better than stainless, so in a multi shot scenario the CM barrel should better be able to dissipate the heat. In a single shot scenario I can see that the tougher oxides from the stainless providing better erosion protection. Go to multi shot and the reduced thermal conductivity will result in higher surface temperatures which may defeat the superior alloy properties.

    There is a reason machine gun barrels are not stainless steel.
     

  3. 25 Otter

    25 Otter Well-Known Member

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    Thanks! Makes sense to me. A stainless big game rig should be just fine if a fella uses his head at the bench. Let her cool while working up loads and go hunting.
     
  4. Canadian Bushman

    Canadian Bushman Well-Known Member

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    No disrespect westcliff, but i disagree. I feel thermal conductivity it very close between carbon steels and stainless and stainless typically has a much higher melting point and less fatigue at higher temperatures, especially the 300 series. Also i believe the manufacturing process in which barrels are made tends to produce slightly better results on stainless rather than carbon based steel. These differences are relative and dont much affect the long range shooter and i think either material will serve the majority of people just fine. I also believe that a stainless barrel will resist throat erosion a little longer than a carbon steel, but again its splitting hairs. I feel the biggest difference is that a ss barrel usually cost more.
     
  5. highridge1

    highridge1 Well-Known Member

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    Stainless barrels are known to last longer .
     
  6. westcliffe01

    westcliffe01 Well-Known Member

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    You "feel" that it is close huh ? Is it the same way one "feels" which feeler gauge fits a gap ?

    If you go to matweb.com and type in 416 into the search window and then look at the properties of some drawn / rolled steel (which is what barrels are) and then compared to a chrome moly you might be surprised.

    Example:
    MatWeb - The Online Materials Information Resource

    416 Stainless Steel, tempered and cold drawn bar 25 mm diameter
    24.9 W/m-K @100C and 28.7 W/m-K @500C

    An alternative 4140 CM (which is what Lilja uses for its CM barrels)
    MatWeb - The Online Materials Information Resource

    AISI 4140 Steel, normalized at 870°C (1600°F), air cooled, 25 mm (1 in.) round
    42.6 W/m-K @100C and 33.0 W/m-K @600C

    So lets see: 42.6/24.9 = 1.71x the thermal conductivity. An increase of 71%. Now a politician might tell you that's not much, but to anyone who cares about science a 71% difference is huge. The difference at 500C hardly matters since the barrel would probably have set your stock on fire already. But 100C is way past the point we should ever let a barrel get to in the first place.

    Chrome moly steel is considerably stronger than stainless steel which is why it is the material of choice for critical fasteners and the like. 400 series stainless steels resist atmospheric corrosion and are better suited to exposure to humidity, condensation and the like, although they are certainly not rust free. 300 series stainless steel are considerably more rust resistant, but have even poorer mechanical properties than 400 series stainless steel. It has a 50% bigger coefficient of expansion, which alone is a major factor in making it unsuitable for anything that is exposed to strong temperature gradients.

    316L, one of the most common "true" stainless steels is listed as having a thermal conductivity of 14.0 - 15.9 W/m-K @0-100C - you will note 1/3 of 4140 CM.

    The fact is that the differences in thermal conductivity between 400 and 300 series stainless steel and CM steel is huge. If you have ever had to build anything that has to reliably operate over a large temperature range, you will discover these differences pretty quickly.

    Speculation in these forums does not contribute to the improvement of the knowledge base. Now I will admit that I speculated regarding 2 different scenarios, where there could be a reason for choosing the one type of material over the other. That's not really speculating, since the military chooses chrome moly for all of its tactical barrels, sometimes with a chrome plated bore to resist rust given battlefield conditions. The chrome will burn off in the throat, since it is very thin, and it does reduce accuracy too. However, there was no speculation on the material properties at all.



     
  7. Canadian Bushman

    Canadian Bushman Well-Known Member

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    I dont want to hijack a thread, nor do i want to make a big stink about something that i dont feel is relavent to otters initial question about throat erosion in the 6.5-284. Feel free to P.M. me and we can further discuss metalurgical properties of conventional gun barrels and their application.
     
  8. westcliffe01

    westcliffe01 Well-Known Member

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    For the record, I have a Shilen stainless select match barrel in 416 (6.5x284) on order from Jim at NSS. I have been waiting on it for a while like many others this year.

    My primary reason for picking stainless is that it survives accidental exposure to moisture and condensation a lot more gracefully than CM. Given that most of the hunting season here in MI it is typically 30F or more colder outside than inside one tends to have severe issues with condensation and even frost coating all metal parts of the weapon. With a CM weapon, I would basically have to remove the receiver from the stock every time I get back home to dry it out with a hair dryer and then re-oil it to prevent rust. That sounds like a high maintenance relationship to me. Where I live I can't just leave my rifles on the porch all winter either...

    Knowing the differences between 416 and 4140 I know to let the barrel cool between shots and not to use it for high volume shooting like prairie dogs. Even so I should expect to see accuracy go south by the time I get to 1500 rounds if not before. My similarly equipped 243 AI with another Shilen select match barrel might make it to 2000 rounds. My similarly equipped 308 Win will probably last a long time... There is not really a lot that one can do about these things. Practice with the 308 and spare the others for "real work"...
     
  9. Canadian Bushman

    Canadian Bushman Well-Known Member

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    As far as throat erosion stainless tends to do better because it absorbs heat slower , as well as releasing it, and suffers less fatigue during expansion at moderate temperatures. Even though it is not by much.

    4140 is capable of much higher strength and hardness when tempered, but experiences cracks throughout the surface of the throat as the rapid expansion and contraction during shot strings and begings to erode quickly. As well as its tendancy to react with the very corosive properties of modern powder under high heat and pressure. This is why they are so often chrome lined.

    Although these differences are minor and present it would not keep me from buying a carbon steel barrel.

    Lilja touches on this in their FAQ section
    Lilja Precision Rifle Barrels - FAQ

    Rifle shooter mag also has s good write up on it.
    All About Barrels

    Otter i hope you find this useful.
     
  10. 25 Otter

    25 Otter Well-Known Member

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    Thanks fellas. Good feed back and links.
     
  11. westcliffe01

    westcliffe01 Well-Known Member

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    Owning a 6.5x284 is like driving a dragster. It goes really fast, but its not going to get to 100 000 miles on the odo... Everything in life has its price. If all our equipment was meant to last forever, life would be pretty boring. Sometimes one has to live it up a little...
     
  12. Time Killer

    Time Killer Well-Known Member

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    As a Heat Treater by trade and an owner of a 6.5/284 I went with stainless.
    As to the previous comments about the difference between 300 and 400 series stainless steels, you are correct. They are 2 totally different animals. 300 series being the Austenitic type and can only be work hardened. 400 series is Martensitic type that can easily be hardened through conventional heat treating. Everything the Bushman said was correct, but I chose the SS barrel for the possibility of lessening the throat erosion problem of this speedster of a cartridge.
    On a less geeky note, it sure is fun to shoot!