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Warm/Cool barrel group differences

 
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  #1  
Old 02-07-2011, 08:43 PM
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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?
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  #2  
Old 02-07-2011, 10:26 PM
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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.
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  #3  
Old 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!!
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  #4  
Old 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.
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  #5  
Old 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.
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  #6  
Old 02-08-2011, 03:47 PM
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Join Date: Aug 2010
Posts: 54
Re: Warm/Cool barrel group differences

Have it cryoed and it will stop doing that.
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  #7  
Old 02-08-2011, 05:20 PM
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Join Date: Aug 2010
Posts: 110
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
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