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affect of ambient temp on velocity?

 
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  #1  
Old 05-13-2013, 05:51 PM
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Location: spokane wa
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affect of ambient temp on velocity?

Can someone help me out with this? I have shot at 75 degrees and 50 gegrees with the same dpoe and hit same spot under moa difference. What % should I put into my ballistic calculator? Running strelok. Also shooting a 300 rum with 215 Berger's at 3040 fps according to drops ( I don't have a chrono) using 90.2 grains retumbo. I have % change per degree at 0.3% now. Just wondering what other guys use for that value.
Thanks
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Old 05-13-2013, 07:16 PM
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Re: affect of ambient temp on velocity?

Quote:
Originally Posted by BULLBLASTER View Post
Can someone help me out with this? I have shot at 75 degrees and 50 gegrees with the same dpoe and hit same spot under moa difference. What % should I put into my ballistic calculator? Running strelok. Also shooting a 300 rum with 215 Berger's at 3040 fps according to drops ( I don't have a chrono) using 90.2 grains retumbo. I have % change per degree at 0.3% now. Just wondering what other guys use for that value.
Thanks
With Retumbo I shoot on a balmy 85 to low dry 20's with out virtually any point of inpact change. It is ashame I have had to change but N570 seems a bit better with good stability as well.
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Old 05-13-2013, 10:27 PM
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Re: affect of ambient temp on velocity?

The amount of change of drop per degree of temp change is dependent on the BC of the bullet. For most bullets in the .6 or better range that will equate to about 3/4 MOA which is 3/4" @ 100 yds and 8" @ 1000 yds with 25* change in temp like between 50 and 75.
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Old 05-14-2013, 12:22 PM
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Re: affect of ambient temp on velocity?

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Originally Posted by MontanaRifleman View Post
The amount of change of drop per degree of temp change is dependent on the BC of the bullet. For most bullets in the .6 or better range that will equate to about 3/4 MOA which is 3/4" @ 100 yds and 8" @ 1000 yds with 25* change in temp like between 50 and 75.
I understand that for bullet flight due to density differences of air at different temps.
I was asking about start velocity like say at 20 degrees f the velocity is 2950 and at 80 degrees f the velocity is 3050. Exaggeration I know but strelok has an input for how much ambient temp affects muzzle velocity. Just not sure how I'd figure it out without waiting until winter and shooting over a chrono.
Does the nightforce program account for this? Or the other phone based ones?
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Old 05-14-2013, 10:18 PM
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Re: affect of ambient temp on velocity?

Every powder will display different temperature regression results. Some more than others. Sorry I can't help you out much other than suggesting shooting thru a crony at extreme temperature variations to verfiy and establish data for a regression solution. Many ballistics engines allow for turning on or off data regression. I mostly use Hodgdon Extreme powders and zero in temperatures near 40 deg F, an average hunting temperature for my region. This seems to work well enough to where I don't need to consider data regression for a firing solution. If I'm shooting Reloader, a different situation where data regression should be considered for long range first round hit probability.
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Old 05-14-2013, 10:31 PM
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Re: affect of ambient temp on velocity?

Quote:
Originally Posted by BULLBLASTER View Post
I understand that for bullet flight due to density differences of air at different temps.
I was asking about start velocity like say at 20 degrees f the velocity is 2950 and at 80 degrees f the velocity is 3050. Exaggeration I know but strelok has an input for how much ambient temp affects muzzle velocity. Just not sure how I'd figure it out without waiting until winter and shooting over a chrono.
Does the nightforce program account for this? Or the other phone based ones?
As MMRES said, starting velocity will be different for each powder type. Some are more temp sensitive than others. I've read some reports that some powders will change about 1 fps per 1* of temp change.

I think the best thing to do is shoot under different temps and record the results. I also think it's a good idea to shoot and confirm velocity and drops during hunting season temps. Here in Montana, temps can vary greatly during hunting season. Probably a good idea to to have several trajectories loaded into the calc based on temp.
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  #7  
Old 05-15-2013, 12:09 AM
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Re: affect of ambient temp on velocity?

There are two separate parts to the temperature equation here.
Air density and powder temperature.

The ballistic program is calculating for air density. Temperature changes the air density.

The ballistic program can not help you with changes in velocity as a result of powder temperature. This data need to be known by you and factored in to the correction independently from air temperature/density. Depending on the range you are shooting you may want more corrective information that 1 fps per 1 degree of temp.

Changes in powder temperature quite often move POI horizontally as well as vertically. This is why experienced shooters keep a shot data log book. My 300Win moves about 1/2 MOA left for a 30 degree increase in temperature.
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