Warm/Cool barrel group differences

Discussion in 'Long Range Hunting & Shooting' started by WV Sendero, Feb 7, 2011.

  1. WV Sendero

    WV Sendero Well-Known Member

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    I have a 22-250 Model 700 VTR w/Bell & Carlson medalist stock, Warne rings and bases, and a Bushnell 6500 2.5-16. I have been trying to work up a load for this gun that would shoot better than 1" 5 shot at 100yrds. I have found about 3 that would shoot right at 1". Well today while trying one of the 1" loads against 2 new ones I discovered that the gun shoots much better on a warm barrel. I had 10 shots left with the 52 grain berger target bullets that had previously shot 1" 5 shot so I decided to shoot them all in a row. The first 3 shots were looking like they were going to be about 1" again but then I shot the next 7 and they were about 1/2" left and in a group that measured 0.48". So why would this gun shoot 7 shots at 0.48" on a warm-hot barrel but only shoot a 5 shot 1" on a cool barrel?
     
  2. Joaquin B

    Joaquin B Well-Known Member

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    WV,

    The phenomenon that makes your rifle shoot more precisely as the barrel heats-up after a few shots is "thermal dimensional stabilization". In plain English, when a barrel is machined and rifled at the factory, some localized heating will always occur. Heat can be minimized by abundant use of cutting fluid and machining using very shallow cuts per pass.

    When machining is aggressive or not enough coolant is used, residual stresses from the heat generated by friction from machining/rifling operations are induced in the steel. As the barrel cools down and the barrel contracts, there will be some very slight deformation of the bore diameter along the barrel's axis, due to these residual stresses.

    When you shoot the first 3 or 4 rounds from a cold barrel, the heat from the burning powder is absorbed by the barrel, causing it to expand. As the barrel expands, the small amount of deformation that resulted from the residual stresses from machining is countered, eliminating the distortion, thus making the barrel more dimensionally uniform, which is why, in your rifle's case, better groups are produced with a warm barrel.

    The key to maintaining good dimensions on machined products lies mostly in heat management. Aggressive machining produces more heat from friction. Relatively fast speeds, very shallow cuts and abundant cooling/cutting fluid flow will result in less residual stresses and tighter tolerances. Stress relieving by placing the machined barrel in an oven for a few hours at moderate temperature before drilling or rifling will also help. Some barrel makers stress-relieve their barrels after rifling them, prior to lapping.

    I hope this helps to explain what you are observing
     
    Last edited: Feb 8, 2011

  3. RDM416

    RDM416 Well-Known Member

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    Joaquin B

    Thanks for your reply to WV... I have seen this explained before, but not as well. It sounds like you have some experience with high precision machining. Good info!!
     
  4. rscott5028

    rscott5028 Well-Known Member

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    great answer!

    I thought for years that you had to have a heavy barrel to shoot good groups without a lot of cooling in between.

    Then, I traded for a Model 70 with a Shilen barrel that isn't a particularly heavy contour. And, it just doesn't seem to care. I guess it was properly stress relieved.
     
  5. WV Sendero

    WV Sendero Well-Known Member

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    Thanks for the great explanation. I always appreciate information from more experienced shooters.
     
  6. ubettcha13

    ubettcha13 Well-Known Member

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    Have it cryoed and it will stop doing that.
     
  7. joejo

    joejo Well-Known Member

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    i have quite a few years experience with the 22-250. fist just let me ask what load you are shooting. ie: powder/bulet/primer sizing process and barrel legnth. i bet i could assist you in getting the gun to 1/2 moa or just slightly over. pm me if you would like
     
  8. WV Sendero

    WV Sendero Well-Known Member

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    52gr Berger target, 34gr 4064, Federal LR match, neck sized, 22" ported barrel so effective barrel length is 22" (Rem 700 VTR). Next best load (best cold barrel load) was with 55gr ballistic tips with 36.3gr 4064. I shot some 40gr Sierra Varminters and they were by far the worst shooting in this gun. Twist is 1 in 14".
     
  9. Natty Bumpo

    Natty Bumpo Well-Known Member

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    I don't know enough about barrels to agree or disagree with what's been offered as explanations. My only question is this: Have you demonstrated this warm barrel phenomenom more than once, or is it all based on the 7 shots you fired once into < 0.5"?
     
  10. Michael Eichele

    Michael Eichele Well-Known Member

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    This is the best explanation of this problem I have ever heard. Thanks.

    BTW, I have seen this a time or two. All have been with factory barrels. Never had a problem with it in a custom barrel.
     
  11. 4xforfun

    4xforfun Well-Known Member

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    I am with "NATTY"

    Shoot 5 groups on a warm bbl, not just 1 group. Then it will be more than just a guess as to weather you have "stress" in your tube.
     
  12. Michael Eichele

    Michael Eichele Well-Known Member

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    I dont speak for anybody else here but I can for that in my experience with a couple of barrels that demonstrated this, I could count on it every time.

    One of the best examples of this I have is a MarkV 30-378 I used to have that would put the first of 2 bullets in a 2 bullet group anywhere and everywhere on a target at 200 yards. Many times as much as 6 or more inches from the middle. The second shot would hit the middle. Let her cool for 15 minutes, start over. The first was anyones guess, the second, right in the middle. After doing this 4 times I had a perfect little nickel size hole in the middle of the target. Too bad I couldnt get her to do that with the first of 2.

    Another load I had would shoot sub 1" 3 shot groups at 300 yards after firing 3 or 4 rounds.

    This was the consistent story of that rifle for the duration of my ownership. After 6 months of pulling my hair out, I sold the POS. It was scary accurate after it was warmed up. Before that the misses were huge.

    M