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Thread: Density Altitude shooting.

  1. #46
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    I have been contemplating for a while now if I must add my findings as well.

    I have been shooting big rifles for a couple of years now and from one shooting location to the next, the same phenomena is encountered. It is not just noticeable with air rifle shooting. I have been battling with this ever since. At 100- 150m there is normally no significant change to justify a change in scope settings. Further than that it becomes an issue and at distance further than 400m it becomes a major factor.

    This is about in proportion to shooting at 15 - 18 m with an air rifle where atmospheric conditions have very little impact but at 20 - 25m it becomes noticeable and at distances further than 30 m you have to do something about it otherwise you miss.

    Last year I bought myself a Kestrel 4500 to be more precise in my medium to long range shooting and to use it for air rifle shooting predictions as well.

    Saturday past, we shot the Presidents shoot on Waterval shooting range in Rustenburg and it is the first time I went shooting there. I used my Kestrel 4500 to measure the conditions and then I did something that I see no one has mentioned in this thread before and what I found to be a very exact way of calculating beforehand what and how your trajectory is going to be.

    The most crucial thing to influence external ballistics is the wind and air density. Now let us leave wind out for a moment and concentrate on air density. Air density is determined by atmospheric pressure, humidity and temperature. Up to now I have seen everybody realize there is a correlation between these 3 factors and the one goes up or down the POI shifts and so on.

    Now as SD has said a little earlier, keep it simple. Now here is a simple solution to accurately know what your rifles performance will be at certain atmospheric conditions.

    Firstly the correlation between these 3 factors is given in a formula:-

    Air density in kg/m? is calculated using the formula AD = Air pressure/(Specific gas constant x Temp) where:

    Air pressure = Pascal (29.53"Hg = 100 000Pa)
    Spec gas constant = 287.05 for dry air
    Spec gas constant = 293 for 100% humidity
    Temp = Kelvin 273 + Celsius

    Let's use an example

    Air pressure is 102900 Pascal
    Humidity is 50%, then use a figure approximately halfway between 287.05 and 293, that is 290
    Temp is 28? Celsius

    Lets calculate AD = 102900/(290x(273+28)) = 1.1788kg/m?

    This 1.1788 is the factor you use for a specific range of conditions that gives you that calculated value. If the temperature changes, humidity changes or Air pressure changes to such an extent that the calculation still gives you 1.1788, THAT is the base figure you use for the external performance or bullet path determined and affected by the AIR DENikko StirlingITY prevalent at that time.

    Yesterday at Waterval shooting range, I took all the readings early when I got there to determine a factor. The humidity went down from 76% to about 22% during the day, temp went up from 24 - 34 , air pressure went up from 105500 to 107110Pa. The actual effect of the air density prompted me to adjust an extra click on 300m shots to still hit home. All the other guys shooting next to me shot low. A friend of mine took my warning to heart to adjust an extra click, which he did and he shot a very good score. If he did not he would have missed.

    Now how can you make these figures work for you?

    What I do is I use a very good ballistics program written by Lapua to calculate a couple of ballistic tables with a variety of different atmospheric conditions. I get a couple of Air density values and I use them as a factor, starting from 1, 1.1, 1.15, 1.2, 1.25, 1.3. At a specific venue I will take the atmospheric condition measurements, calculate the Air density and from my cheat sheets I will select the right drop chart and shoot accordingly. So far I have seen it work for shooting at 5800ft in the Drakensberg mountains, Genl Piet Joubert, Heidelberg, Rustenburg, Delmas and a couple of place where I have put my hand to long distance shooting up 1000m. It worked every time to the click.

    I have tried it with the air rifle as well and it also works in the same way. The change in conditions as measured above will cause you to miss a 40mm kill zone even with no wind @ 50m. The change in conditions was measured in a time span of 2 hours. So if you shoot in early and zero your rifle and only start shooting an hour or 2 later and with changing conditions you will have POI shift even with the most stable shot to shot velocity.

    How legal it will be to quickly take a measurement and check which cheat sheet you must use during a shoot remains to be seen. It is fairly quick to take the measurement do the calculation and adapt. If you shoot in at say 20m the change in conditions will have no noticeable effect but further than that you may get some erratic and unexplainable behavior.

    This is my findings with what I have been experimenting now for about a year and a half with and for me there is no other way to tackle a new venue or shooting conditions for any place.
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  2. #47
    Protea FT Team '07/'08/'09/'10/'11/'12/'13
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    Francois, I tried to determine a trend in the AD a few years ago, and could not realy come up with a specific trend. The Density Altitude gave me more of a real time trend. My meter gives me a AD figure. I would like to hear more about your program your using
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  3. #48
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    To be honest, Chairgun will do the same thing. The trick is to determine the DA where you are when you sight in.

    Changing venues throws all bets out the window and one must start from scratch. Establish a new "starting set" of values for your sight in/zero at 25 yards. If you do your sighting off a bag/bench all the better for precision.

    If wanting to use a varying set of click charts, I would agree that having a few prepared in advance is a valid method. The trick is to determine the value for DA when you sight in at the new venue. But as I mentioned before, that value is only the starting point to inform us where we are starting from. It is always best done in advance.

    I would agree that just as in shooting inclines/depressions, the effects are not so observable up close, and have greater observable effect out far. They are still occurring from the muzzle to target over the whole trajectory however, and it is only our perception and the scale of shift that appears as a shift. (If we cannot discern it, is it really happening? If a tree falls in the woods and nobody sees it, did it really happen? scenario) Shift can be observed at our primary zero of 25-30 yards though, and using this as the way to rectify a shift is the only real way to get a handle and correct for it, but the ptiy is we cannot take sghting shots in a FT match once begun (sacrificial shot is a good coping strategy if you suspect shift).

    For those with a Kestral or like piece of equipment, you will be able obviously to determine your starting point for DA and zero in for it before a match, and observe changes to DA. It will be only a trend you are observing, but a good accurate relatively "real time" picture of the trend.

    For those of us not in possession of a kestral, the DA chart figured earlier in the thread can do the same task, using temperature only as an indicator. One can determine DA value for your location (in advance) zero the rifle knowing that DA value, and then observe a trend during the match by checking the chart against the temp changes.

    Here's where getting to the ACTUAL values of shift for any given temp shift gets interesting............

    Dave you asked what a 5 degree temp shift would mean?



    Looking at the chart, taking my own location at sea level I have demarked a common temperature range for New Zealand conditions, so a drop of 5 degrees means a drop of 500 feet DA. I have determined that that effects one quarter inch drop at 50 yards (1/8 inch drop at 25 yards), or in other words 1/2 MOA. (1000ft DA change = 1 inch shift at 100 yards) It affects the entire trajectory in a similar manner, but is slightly complicated by the scope height/bore line relationship....so due to the extreme height we set our scopes at in FT, beware the greater gap up real close, and a higher shift out far. Chairgun or any other ballistic program will correctly predict this ONLY if you enter the DA value in the altitude value, and then zero the rifle at its zenith again.

    You will note some red dots on the chart higher up, which represent the Worlds at Mutango last year. Sighting in temps, and match temps for each of the three days with a climb up the hill also plotted.

    If one takes the chart as a clean slate, takes a weather forecast, prepares the day before in similar conditions and time to a match, one can plot a predicted trend............and then observe and monitor temperatures alone, or DA change using a kestral to track changes to match them to a predicted trend.

    How you personally sight in or handle the changes is up to individual shooters. Sighting in the middle of an expected trend to play a percentage game, or sighting for the expected maximum and then track the change until the rifle "comes on" later in the match, or making changes shot by shot........pre preparing click charts for different conditions.........that is up to the individual to devise a coping strategy. They are all valid strategies.

    I just feel that a kestral is a very useful tool, but not absolutely essential if you understand the environmental process going on, and take the appropriate preparatory steps.

    I hope this is all helping with your preparation for Hungary.

    kind regards
    GS
    Last edited by greyskullnz; 20-09-10 at 02:28.
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  4. #49
    Protea FT Team '07/'08/'09/'10/'11/'12/'13
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    GS, that is a very good piece of work you have done there!
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  5. #50
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    If I had not come to South Africa last year, I would not have been confronted with the shift problem you have there, and I would not have been forced to explain to myself why I saw 4 MOA shift between the first sight-in and the end of the match.

    All other difficulties (self inflicted) aside, I learned much more by attending at Mutango than staying home would have taught me. We don't get more than 1 MOA shift at any stage over here, and many just put it down to marksmanship skill errors. Seeing is believing, and experiencing is thought provoking.

    You guys will have to give some very serious thought to the altered trajectory ARC when in Hungary. Your shift issues may significantly reduce, but your trajectory ARC will significantly increase. Spot on range finding will play a larger part of your success than predicting shifts, but how many others will be experienced with spotting a shift when it occurs? Unknown, but it is the Worlds......and the best will be prepared for anything.

    Now,..........just to throw a spanner in the works,.........which way would you expect the wind to deflect a Diabolo pellet when it blows from the left? Up or down?

    GS
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  6. #51
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    GS and Francois, thanks for your knowlege.

    GS I have watched the wind effects closely, as it was extremely windy here over the last 6 weeks.

    At X 50 magn. a light directional wind has little effect on the vertical movement of a pellet.

    However, once the wind starts to blow me off centre by more than 6 mildots I found vertical poi movement.

    Now heres the strange thing. With my Wally it was hardly noticable but with the EV I clearly see it.

    Strong Wind from R-L will push me down and L-R will push the pellet up, contrary to the standard model presented to us. This L-R effect is amplified greatly when shooting slightly downhill, as can be expected.
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  7. #52
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    Quote Originally Posted by Sniperdad View Post
    GS and Francois, thanks for your knowlege.

    GS I have watched the wind effects closely, as it was extremely windy here over the last 6 weeks.

    At X 50 magn. a light directional wind has little effect on the vertical movement of a pellet.

    However, once the wind starts to blow me off centre by more than 6 mildots I found vertical poi movement.

    Now heres the strange thing. With my Wally it was hardly noticable but with the EV I clearly see it.

    Strong Wind from R-L will push me down and L-R will push the pellet up, contrary to the standard model presented to us. This L-R effect is amplified greatly when shooting slightly downhill, as can be expected.
    Very interesting that you have found contrary to conventional thinking on this subject.

    Photos below credited to Harry Fuller of Australia. (He's the sub MOA 100 yards fella)



    When trying to confirm his belief that Diabolo pellets behave contrary to cylindrical bullets, he shot a comparison set in identical conditions.



    His examples of Diabolo deflection at top, and cylindrical shaped "bullet" type pellets at the bottom of the Frypan.




    Credit to Harry, he felt the same way you did that the standard theory did not apply, stuck to his guns and tested his theory.

    I think its a good starting point for the rest of us to either prove it or disprove it ourselves.......and also determine if it actually occurs with .177 cal, as I have never seen it even with extreme wind movement of the pellet. Its always horizontal from the Steyr, even out past the mildots into the thick part of the x hairs.

    This has implications for those switching back and forth between JSB Diabolo design and JSB Heavies which are pretty much cylindrical.

    cheers
    GS
    Last edited by greyskullnz; 20-09-10 at 12:24.
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  8. #53
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    Damn, I wish I could just pull the trigger and hit the target - I though golf could get confusing. FT takes a lot more skill, practise and experience!
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  9. #54
    Sharp Shooter
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    Thanks guys for a most informative thread. been a while since a controversial topic like this was discussed without it being derailed. And special thanks to Windbuksie. Your formula cleared it up for me. Now I know what to track and keep an eye on and understand the relationship between them.
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  10. #55
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    My pleasure JT !

    And yes, I'm verry happy with the amount of discussion this has caused.

    It is great to see the way the old hands think !

    Every second post gave me food for thought.
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  11. #56
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    QUOTE GS//"Credit to Harry, he felt the same way you did that the standard theory did not apply, stuck to his guns and tested his theory.

    I think its a good starting point for the rest of us to either prove it or disprove it ourselves..."


    I tested it again this afternoon, results confirmed, but it only applies in strong wind.

    My wife tagged along ans shot her Steyr and found the same results, canning my idea that its an EV thing.

    R-L=lower
    L-R=higher(from 9h00)
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  12. #57
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    Quote Originally Posted by Sniperdad View Post
    QUOTE GS//"Credit to Harry, he felt the same way you did that the standard theory did not apply, stuck to his guns and tested his theory.

    I think its a good starting point for the rest of us to either prove it or disprove it ourselves..."


    I tested it again this afternoon, results confirmed, but it only applies in strong wind.

    My wife tagged along ans shot her Steyr and found the same results, canning my idea that its an EV thing.

    R-L=lower
    L-R=higher(from 9h00)
    Are we sure it is not a Southern Hemisphere thing? Plugholes and vortexes and the suchlike turning in reverse?

    .................. just joking

    GS
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  13. #58
    Rest in Peace Mike 05/12/2014

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    Some of the barrels are reverse rifled for the Southern Hemisphere and ......
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