affect of ambient temp on velocity?

Discussion in 'Long Range Hunting & Shooting' started by BULLBLASTER, May 13, 2013.

  1. BULLBLASTER

    BULLBLASTER Well-Known Member

    Messages:
    83
    Joined:
    Feb 6, 2012
    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
     
  2. porkchop401

    porkchop401 Well-Known Member

    Messages:
    298
    Joined:
    Dec 19, 2010
    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.
     

  3. MontanaRifleman

    MontanaRifleman Well-Known Member

    Messages:
    6,068
    Joined:
    May 21, 2008
    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.
     
  4. BULLBLASTER

    BULLBLASTER Well-Known Member

    Messages:
    83
    Joined:
    Feb 6, 2012
    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?
     
  5. MMERSS

    MMERSS Well-Known Member

    Messages:
    912
    Joined:
    Feb 5, 2013
    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.
     
  6. MontanaRifleman

    MontanaRifleman Well-Known Member

    Messages:
    6,068
    Joined:
    May 21, 2008
    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.
     
  7. X-man

    X-man Well-Known Member

    Messages:
    268
    Joined:
    May 4, 2007
    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.
     
  8. MMERSS

    MMERSS Well-Known Member

    Messages:
    912
    Joined:
    Feb 5, 2013
    X-man,

    Great reply with noted experience. As you pointed out, the internal ballistics due to velocity changes can be significant.

    Many times during load development I have adjusted the powder charge by .2 grains. Group ES with the different powder charges and group size may be similar however the POI between the two groups sometimes have more of a horizontal correction as much as vertical due to the change in velocity and other factors causing the changed behavior to the internal ballistics of the gun. Good comment addressing powder temp, velocity data regression and internal ballistics all being interrelated. If not correcting for the difference in temp/velocity as you have noted, keeping your powder temp while hunting as close to your zero temp, assuming your powder temp has adjusted to the outside temperature during zero, and using a less temperature sensitive powder such as Hodgdon Extreme can be an alternative.

    One simple technique to compensate for significantly colder field temperatures than your zero temp is keeping your ammo or magazine in an outside breast pocket. Your body temperature will warm the ammo or magazine increasing the powder temp above outside temperatures. You can also place your Kestrel inside the same pocket to verify the powder (pocket) temp.