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Testing Loads? Help

 
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  #1  
Old 02-27-2012, 07:13 PM
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Join Date: Oct 2007
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Testing Loads? Help

Getting your guys thoughts on testing loads for accuracy when it is colder, It seems like I'm getting different results when the temps are in the teens compared to 40's, Just not getting the accuracy I think should, maybe it is the temps on the rifle also, just give me your oppinons
thanks
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Old 02-27-2012, 08:21 PM
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Join Date: Jan 2010
Location: Leominster mass, originally Salisbury Maryland
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Re: Testing Loads? Help

It is largely due to pressure variations, versus pressure at initial load creation. Better said, at any temp, when the gun and load are TUNED together, you have to remember that under those conditions, that is where it will shoot best. When the temp drops, the pressure created by the burning powder decreases due to temp resistance. When the temp rises, so does your pressure, and sometimes to the extreme. Temp also effects the way the barrel oscillates. As the temp lowers, the stiffer the steel gets, and the warmer, the softer it gets. When you complex those two things alone, you can get a better picture on cause and effect. Sometimes it seems as though everything is working against you. The worst case scenario happened to me this past hunting season. When I did my load development, it was in the fall. The weather was at it's usual for that time, and hunting season was 2 weeks away. When I got down to Maryland, the temp was in the 70's. When I did my spot shooting, the day before, I had to lower all of my settings due to higher pressures. Record keeping is paramount in this sport. When conditions change, so does consistency. Hope this helps.

7STW.
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Old 02-27-2012, 09:18 PM
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Re: Testing Loads? Help

I've never seen any temperature, humidity or any other atmospheric condition change how my long range match rifles' accuracy changed. While temperature effects how the powder burns, as long as it doesn't change drastically between shots, it's not a concern. But if you leave a round in a hot barrel too long, it will shoot the bullet out faster, so fire the round within 15 to 20 seconds of chambering it with hot barrels.

And barrel steel doesn't change its fundamental vibrating frequency with temperature. At least in those they humans stay alive in. If they did, then the mechanical engineering 4th order equations to calculate how stiff they are would have a correction factor for temperature; there aint' one.
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Old 02-28-2012, 05:17 PM
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Location: Allen, TX
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Re: Testing Loads? Help

Some powders are more sensitive to temperature changes than others.

And, if your load is borderline relative to pressure/velocity, it could make a slight difference.

Also, air density may have a marginal impact if you're borderline.

Like Bart B said. If you're load is really good, then temperature alone shouldn't be an issue.

On the other hand, metals, wood, plastics, etc expand/contract differently according to temperature/humidity. If your barrel isn't properly floated with say a wood stock, and/or your bedding isn't right, you could be having an issue with the rifle itself.

Without more info or being there, we're just speculating.

-- richard
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Old 02-28-2012, 05:50 PM
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Join Date: Jan 2010
Location: Leominster mass, originally Salisbury Maryland
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Re: Testing Loads? Help

Quote:
Originally Posted by Bart B View Post
I've never seen any temperature, humidity or any other atmospheric condition change how my long range match rifles' accuracy changed. While temperature effects how the powder burns, as long as it doesn't change drastically between shots, it's not a concern. But if you leave a round in a hot barrel too long, it will shoot the bullet out faster, so fire the round within 15 to 20 seconds of chambering it with hot barrels.

And barrel steel doesn't change its fundamental vibrating frequency with temperature. At least in those they humans stay alive in. If they did, then the mechanical engineering 4th order equations to calculate how stiff they are would have a correction factor for temperature; there aint' one.
P/ S, I forgot to add, IMHO.........7 STW
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Old 02-28-2012, 06:09 PM
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Posts: 2,114
Re: Testing Loads? Help

Quote:
Originally Posted by rscott5028 View Post
Like Bart B said. If you're load is really good, then temperature alone shouldn't be an issue.

On the other hand, metals, wood, plastics, etc expand/contract differently according to temperature/humidity. If your barrel isn't properly floated with say a wood stock, and/or your bedding isn't right, you could be having an issue with the rifle itself.
Plastics and other synthetic stock materials had problems when they first came out. When used on M14NM's and M1A's, they warped enough in the hot sun laying atop ones shooting bag to cause a 2 MOA change in impact at all ranges. With those stock's front end fitted to a ferrule on the barrel and receivers well epoxy bedded, the pressure on the barrel played hell with zeros. Some folks reported that after starting a string of 20 shots in prone at 600 or 1000 yards, they went through a 2 MOA change in zero; sometimes more. Fortunately, the later synthetic stocks were better. Never seemed to have that problem with wood stocks heating up in the July afternoon sun.

And yes, clearance around the barrel in a bolt gun is critical. One needs at least 1/16th inch and 1/8th is better. Winchester made some of their sporter stocks' fore ends with a gap wide enough (3/32nds?) to keep warping ones from putting pressure on the barrel. But too many folks thought they looked too ugly and/or was afraid something would get down in between the fore end and the barrel and cause problems. Winchester quit doing this; they chose to cater to folks who thought a perfect, tight close fit was best.........
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