Air temperature and velocity changes.

Discussion in '6BR, 6BR Improved & Wildcats' started by jackson1, Jan 16, 2019.

  1. jackson1

    jackson1 Silver $$ Contributor

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    Tell me if I am making an incorrect inference. Currently shooting in 20F to 30F degree temperatures up here in the great white north (brown), in preparation for the up coming 600 yard season. My observation is that there is a correlation between velocity and accuracy and as the temperature increases my charge weight will need to decrease, in order to maintain a given velocity. Am I looking down the wrong rabbit hole, by assuming that maintaining a certain velocity, will lead to good accuracy through out the season?
     
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  2. BRGUY

    BRGUY

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    Given a specific gun, load and velocity @ 20 to 30 degrees, does NOT mean that the same "velocity" will be accurate at say 85 to 95 degrees.

    I have developed loads in 30 degree and colder weather for long range winter coyotes, plus using a magnum primer. Then in the summer time, I drop the magnum primer and decrease the powder charge 1.5 to 3 grains to achieve the same accuracy level, but velocity would be off by some 175 fps. or more.

    Caliber, cartridge, barrel length, bullet weight, load, powder, temperature, elevation, etc., etc., all make a difference...
     
  3. jr600yd

    jr600yd Gold $$ Contributor

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    Jackson1,
    Your assumptions are correct. How much of a change in your rifle is the question. The only way is to work up loads in the cold and again in warm weather. Anything else is just a quess.
     
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  4. XTR

    XTR

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    That is a challenge for any of us who shoot competition.

    If you've changed things since last yr that keep your last load from working, then developing a new load in temperatures 50 to 60 degrees colder than those that you will actually see in competition is less than ideal. This is esp true if you end up running close to the limit on pressures. You really don't know what is going to happen until you get out there and shoot in the 80s.
     
  5. dmoran

    dmoran Gold $$ Contributor

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    @jackson1
    Charting your achieved velocity's to the ambient, is a good way to establish powder sensitivity to your own scenario's.
    Maintaining a velocity range, by adjusting your powder charges to the ambient temperature, is a good way to stay in tune.
    Attaching a chart below I made/used several years back for N140 / 6Dasher:

    DasherChart140.jpg
     
    Last edited: Jan 16, 2019
  6. oldduc

    oldduc Silver $$ Contributor

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    I bet it gets colder than 30F in North Dakota. Though, actually, when it gets colder than 30 I usually just shoot the bull rather than the Dasher.
     
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  7. mike a

    mike a Silver $$ Contributor

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    I'm thinking the firing pin might move rather slow in freezing temps. I saw them not move at all in Alaska. Sub zeros.
     
  8. jackson1

    jackson1 Silver $$ Contributor

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    Thankyou dmoran, a chart is a great idea and just the response I needed.
     
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  9. jackson1

    jackson1 Silver $$ Contributor

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    oldduc, I am in Wisconsin, located between Green Bay and Milwaukee on the lake. No 20's and 30's are not all that cold. The older I get, the less inclined I am to shoot in cold weather. Just a few years ago my lower limit was 10 degrees, have now moved into the 20's.
     
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  10. comagutsa

    comagutsa

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    here is one for ya with my 6BRX and 32.2gr of 2208/varget and 105gr hybrids, i shot on the weekend starting temp was approx 16C/60F over the next 2 hours it got to 30c/86f, 5 targets in that time, these are avg speeds
    1st 2956fps ES 5 SD 2.5
    2nd 2970fps ES 3 SD 1.5
    3rd 2974fps ES 8 SD 3
    4th 2978fps ES 12 SD 4.5
    5th 2982fps ES 6 SD 2.4
     
    Last edited: Jan 18, 2019
  11. oldduc

    oldduc Silver $$ Contributor

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    Well, you're doing better than me. I shot at around 30 degrees a couple weeks ago and the lube in my front rest was freezing and not letting it keep up with the tension springs and my velocities were way down and my butt was cold on the stool and I couldn't feel the trigger and the sun got in my eyes and the wind was blowing and etc...... I'm sure there are also other excuses why I shot worse than usual. I'm thinking 45 is a good minimum temp. Good thing I don't live any further north. :)
     
  12. topclass2017

    topclass2017 Silver $$ Contributor

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    great chart Donovan, and a timely discussion. I've been thinking the same as the OP - how will velocity change with changing temps? Is it straight line? is there a rule of thumb? how much do I need to alter my charge weight based on temps? Are some powders more temperature sensitive than others? I know that the only way to get good empirical answers to these questions is to go out and test. and test. and test some more so as to develop charts like the one Donovan showed.

    I am in Central Texas so there are some rather extreme temperature ranges to address. Yesterday temps were in the upper 60's. A week ago high 30's/lower 40's. And summers...I know I cannot use winter loads in the summer (the other way around is less likely to cause a problem, other than potential for accuracy loss) but it is good to have a blueprint for determining just what adjustments should be to ensure a day at the range is enjoyable, accurate, and safe.

    I guess that is one of the reasons I enjoy re-loading - it appeals to my scientific curiosity. I am not a competitive bench shooter but I do like to wrangle the best out of my rifles.
     
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  13. seymour fish

    seymour fish

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    . Don’t rely on it being a linear function
     
  14. dmoran

    dmoran Gold $$ Contributor

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    If you look closely at the data in the chart, you will see it is not representing linear function.
    And IME, have never seen any powder to react linearly to temperature (at least not across a wide range of temperature).
     
    Last edited: Jan 28, 2019
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  15. jackson1

    jackson1 Silver $$ Contributor

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    Seymour fish, Thankyou for your comment. I didn't expect my charge weights or velocities to be linear. That is why I test. From conversations with other respected shooters and this forum, I kind of arrived at the notion that there is a velocity sweet spot for a given bullet. With that in mind, I am attempting to develop a chart of temperature and velocity, to attempt to stay in that sweet spot. I plan on loading 24 hours prior to a match and that should give me an accurate expected temperature on which to set my charge weights. May be climbing down a rabbit hole, but I'm having fun. I know that there are more variables than air temps but I have to start some where. Who knows, next year, I may be chasing a whole new idea. The beauty part is, the chase never ends.

    Can't do every idea or shooting type I get to read about here, but I enjoy the exposure to them I receive on this site.
     
  16. seymour fish

    seymour fish

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    Agree 100% with your last statement. Knowing where the radical departures lie is critical and would make an informative topic re:various commonly used powders.
     
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  17. 4xforfun

    4xforfun Gold $$ Contributor

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    Keep you gun warm.....by shooting. Keep your ammo warm. Your gun and ammo don't know that it is 20 or 80 outside. Keep them at summertime temps and groups should be the same.
    The only changes will be the shooter/ rifle combo (you will have more bulky cloths on ) so it may be harder to shoot well and point of impact due to denser air. Groups will not change shape or size.

    Now, let you gun, ammo, or both get cold and it all goes out the window. Then you run into speed change which changes harmonics. That is where info like Donovan gave you REALLY comes in handy.

    Tod
     
    Last edited: Feb 15, 2019
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  18. jackson1

    jackson1 Silver $$ Contributor

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    Waiting on the snow to quit and then shovel it. Forecast says 20 below at night till Friday. Rain for the weekend. Will spend the week in the basement loading something.
     

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