close
Long Range Hunting Online Magazine


Go Back   Long Range Hunting Online Magazine > Hunting > Long Range Hunting & Shooting

Long Range Hunting & Shooting Nightforce Optics

Reply

Warm/Cool barrel group differences

LinkBack Thread Tools Display Modes
  #1  
Unread 02-07-2011, 08:43 PM
Silver Member
 
Join Date: Jan 2011
Location: West Virginia
Posts: 169
Warm/Cool barrel group differences

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?
Reply With Quote
  •   #2  
    Unread 02-07-2011, 10:26 PM
    Silver Member
     
    Join Date: Sep 2009
    Location: Oro Valley, Arizona
    Posts: 127
    Re: Warm/Cool barrel group differences

    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
    __________________
    Joaquin B. in AZ

    Last edited by Joaquin B; 02-08-2011 at 02:33 AM.
    Reply With Quote

      #3  
    Unread 02-07-2011, 10:35 PM
    Gold Member
     
    Join Date: Jan 2005
    Location: Searcy, Arkansas
    Posts: 700
    Re: Warm/Cool barrel group differences

    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!!
    Reply With Quote
      #4  
    Unread 02-07-2011, 10:36 PM
    Platinum Member
     
    Join Date: Apr 2010
    Location: Allen, TX
    Posts: 2,608
    Re: Warm/Cool barrel group differences

    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.
    Reply With Quote
      #5  
    Unread 02-07-2011, 11:00 PM
    Silver Member
     
    Join Date: Jan 2011
    Location: West Virginia
    Posts: 169
    Re: Warm/Cool barrel group differences

    Quote:
    Originally Posted by Joaquin B View Post
    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 dimensianally 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
    Thanks for the great explanation. I always appreciate information from more experienced shooters.
    Reply With Quote
      #6  
    Unread 02-08-2011, 03:47 PM
    Bronze Member
     
    Join Date: Aug 2010
    Posts: 54
    Re: Warm/Cool barrel group differences

    Have it cryoed and it will stop doing that.
    Reply With Quote
      #7  
    Unread 02-08-2011, 05:20 PM
    Silver Member
     
    Join Date: Aug 2010
    Posts: 113
    Re: Warm/Cool barrel group differences

    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
    Reply With Quote
    Reply

    Bookmarks

    Thread Tools
    Display Modes


    Similar Threads for: Warm/Cool barrel group differences
    Thread Thread Starter Forum Replies Last Post
    Group differences in Weatherby Brass versus new Norma Brass TX mountain hunter Long Range Hunting & Shooting 5 12-29-2010 10:17 PM
    Cold bore shot group vs Warm bore shot group Alan Griffith Rifles, Bullets, Barrels & Ballistics 24 06-07-2010 11:58 AM
    Warm up your Ballistics Calculator! bigngreen Rifles, Bullets, Barrels & Ballistics 14 03-05-2010 11:30 PM
    barrel cool-down skandlus Rifles, Bullets, Barrels & Ballistics 7 09-20-2009 08:36 AM
    good warm weather backpac boots 264win Backpack Hunting 12 05-11-2009 01:24 PM


    All times are GMT -5. The time now is 06:42 AM.


    Powered by vBulletin ©2000 - 2016, Jelsoft Enterprises Ltd.
    Content Management Powered by vBadvanced CMPS
    All content ©2010-2015 Long Range Hunting, LLC