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SS vs CM .308 Barrel?

Discussion in 'Rifle Discussion' started by acdodge, Aug 22, 2014.

  1. acdodge

    acdodge Hillsboro/ Wilsonville Member

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    So I have a Remington .308 AR that has a 20" fluted CroMoly Barrel. I get approx. .75 MOA with it shooting from a bipod with match grade ammo. The issue I am having is that when I shoot fairly rapidly the barrel heats up and starts opening up my groups to about 2.5 MOA.
    The question is would a Stainless barrel help with heat dissipation and/or help to keep my groups close? I know CL barrels are made for better heat dissipation but you sacrifice accuracy for it.
     
  2. Darkker

    Darkker Mesa, Wa Active Member

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    No...
    The reason your accuracy diminishes is due to stress in the barrel.

    Stainless steel does not transmit heat as quickly as chrome-Moly, meaning it will not dissipate heat as quickly. What stainless barrels are typically better at, is they work harden, so throat erosion usually happens slower. But a poorly made barrel is poorly made, regardless of material.

    Chrome lined barrels are NOT built for heat dissipation. They are built because of how hard they are, further slowing the flame cutting and throat erosion.
    The reason they tend to be less accurate, is the bore must be over-sized prior to chrome deposition. How uniform that chrome job is, is what affects accuracy.
     
  3. erudne

    erudne The Pie Matrix PPL Say Sleeping W/Your Rifle Is A bad Thing? Bronze Supporter

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    WW2 standards were a cal 30 rifle barrel must be combat effective to 10,000 rounds (sans salt water and the like). I have fired MOA groups w/new 2 grove 1903A3 barrels, war time production, that I bought new in cosmoline for $10. Quality or material is not the problem.
    Rate of fire is key, especially w/closed bolt designs, many engineers have tried to solve this problem, Open Bolt designs, Water Jackets, nothing beats out thermal dynamics
    My advise is "shoot slower"
     
    Jamie6.5 likes this.
  4. acdodge

    acdodge Hillsboro/ Wilsonville Member

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    So yes/no on SS being better at keeping its form and not deforming or whipping under the heat of semi-rapid fire?

    Also to clarify, rapid fire for me is about a round every 3 seconds or so. After about the seventh round in my 20 round mag is when they start flying astray.
     
  5. Darkker

    Darkker Mesa, Wa Active Member

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    Correct, SS will not dissipate heat faster.

    Stresses (usually from rifling/straightening) are what cause the barrel to "walk" when it is hot.

    So slow your rate of fire. A better quality barrel **shouldn't** walk like that when hot.
     
  6. erudne

    erudne The Pie Matrix PPL Say Sleeping W/Your Rifle Is A bad Thing? Bronze Supporter

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    It sounds like a pressure point affecting the barrel is it free floated?
     
  7. acdodge

    acdodge Hillsboro/ Wilsonville Member

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    It is free floated. It has a Troy MRF 308 handguard on it. It does have a muzzle break on it though I can't see that causing any issues.

    Darkker, Do you have a suggestion of a quality barrel that is more in the line of a working stiff's budget ($600 Rainier arms or noveske barrels are a bit out of my price range).
     
  8. Darkker

    Darkker Mesa, Wa Active Member

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    I just pulled my head out of a dark place.... This is an AR pattern rifle, so none of them are exactly free-floating. You have a gas block and gas tube connecting things. At the least that is going to play with harmonics.
    But no, for AR pattern stuff I can't say I've had enough experience to point you in any one direction. Just that typically factory sporter button rifled barrels tend to do that.
     
  9. Otter

    Otter Oregon - mid Willamette Valley Active Member

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    Far as I know, chrome-moly and stainless will shed heat at the same rate provided they are of the same dimension. So going to stainless will not solve your problem. Fluting or sand blasting either one is about the only thing you can do to help with them shedding heat. Getting a larger diameter barrel will help with absorbing more heat, but not sure that is an option for you.

    CL...I take it that means chrome lined? Lining the barrel with chrome does nothing for heat. It is a way to make the barrel last longer during rapid fire because chrome is harder than either stainless or chrome-moly and makes the lands less prone to erosion when they are hot. I'm sure it doesn't help accuracy but don't know for a fact it hurts. Certainly benchrest barrels are never chrome lined.

    Based on watching people shoot AR style rifles at Tri County, I would say your 3/4 inch groups during slow fire is outstanding. The fact groups open up during rapid fire seems normal to me. Small groups take concentration and shooting fast in my experience never leads to small groups. Add in the barrel distorting as it heats up and I think you are getting a typical result with an AR style barrel.

    I'm not sure this is a problem that can fixed. My bolt action sporter weight barrel rifles even when slow fired tend to shoot larger groups if I shoot more than 3 or 4 rounds at a time. That is why I always shoot 3 shot groups with these rifles rather than 5 shot groups like with my target rifles. You may want to live with the accuracy of the AR as is, and for shooting tight groups get yourself a heavy barrel bolt action rifle.
     
    Last edited: Aug 25, 2014
  10. DieselScout

    DieselScout S Clackamas County Well-Known Member

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    Fluting may hurt you as it is going to leave less material. Less material will heat faster. Fluting is for weigh reduction primarily.
     
  11. acdodge

    acdodge Hillsboro/ Wilsonville Member

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    Thanks for the info guys. From the gist of it, I'm taking that I should just be happy with my current setup. I suppose its just the constant pursuit of perfection that drives us all towards that bullseye. I will definitely take any other sound piece of advice anyone has to offer up... perhaps some special technique for semi-rapid fire, etc. IMAG0456.jpg IMAG0458.jpg IMAG0485.jpg
     
    EVILZ likes this.
  12. DieselScout

    DieselScout S Clackamas County Well-Known Member

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    I am not really sure what the point of "rapid fire" accuracy is. You want pinpoint accuracy take the time to make the shot.
     
  13. acdodge

    acdodge Hillsboro/ Wilsonville Member

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    Ok so here's the scenario... A heard of zombies is closing quickly on my position from 600yds away. Well as we all know you have to make a brain shot. As it turns out I would have to make at most, a rapid succession of 1 MOA shots to take them down before I'm zombie food. Not a likely situation to be in but hey, gotta prepare right?
     
  14. Darkker

    Darkker Mesa, Wa Active Member

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    Thermal conductivity values (W/m-k)
    4140: 42.7
    416: 24.9
     
  15. acdodge

    acdodge Hillsboro/ Wilsonville Member

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    Is that saying that ChroMoly has a better heat transfer rate?
     
  16. Otter

    Otter Oregon - mid Willamette Valley Active Member

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    Been a long time since I took thermodynamics in college. Explain it to a 3rd grader please....

    Using a laser thermometer, I haven't been able to detect one cools faster than the other. But that isn't a very scientific method of course.
     
  17. acdodge

    acdodge Hillsboro/ Wilsonville Member

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  18. Darkker

    Darkker Mesa, Wa Active Member

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    What that means is that it conducts, transfers and loses heat almost twice as fast as 416 stainless.
     
  19. Otter

    Otter Oregon - mid Willamette Valley Active Member

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    That may be true on paper, but in practical experience I don't think it is the case.
    Certainly my stainless barrels don't stay hot twice as long as my chrome-moly barrels. Like I said, with a laser thermometer they seem to cool at the same rate. I will have to test it a little more closely. Could the bluing have anything to do with it?
     
  20. Otter

    Otter Oregon - mid Willamette Valley Active Member

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    I'm trying to find your figures but everything I'm finding indicates chrome alloy steels have a similar heat transfer as stainless steel. Can't find info about specific alloys with the numbers though. Where did you get the numbers?

    Also, wouldn't the reverse be true? If it sheds heat faster, wouldn't it heat up faster as well? If your numbers are correct, and I have no reason to doubt them, chrome moly steel is closer to a blackbody then stainless. If I remember correctly, a black body is a perfect absorber of energy, but also a perfect emitter.