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Thread: confused about trajectories

  1. #1

    confused about trajectories

    Something I have been pondering over and would appreciate insight...

    I have a recently acquired Sako 75 in .243. I put a new scope on it at the weekend, and went to zero it. 90gr Sako ammunition.

    Started at 30m, just to get it on paper. Got it shooting about half an inch high at 30m.

    Moved it to 100m. It was now shooting about 7 inches high. This really surprised me. Would you expect this?

    I zeroed it down to an inch high at 100m.

    Out of curiosity, I moved the target forward again to 30m. Now shooting about dead on (an inch below where it had been to start with).

    I thought I understood trajectories, but this really confused me.

  2. #2
    M. Sounds good to me. A little high at 30m will be a lot high further out. I try and get dead on at 25 yds, then move back to 50 yds and try harder to be dead on. I then expect my 100 yds shot to be about 1 inch high. I check at 100 yds and then terrain permitting check at about 160 yds as I expect the bullet to be dead on again. Finally, location permitting I will attempt a longer range group.

    In doing all this I'm trying to rest/hold the rifle in a consistent prone position. I expect different POIs when I sit or stand.

    Hornady and jmbal have online ballistic calculators. It's interesting to plug your numbers into one of these and I recommend not to mix Imperial and Metric measurements.

    Regards

    JCS
    Last edited by jcampbellsmith; 17-06-2014 at 08:12.

  3. #3
    Mungo,

    The 30 metre point is the first time the bullet crosses the horizontal "still going up", not the second time "when it's coming back down". Parabola, not a straight, rising line.
    At 100 metre you were still on the top of the parabola until you adjusted it down to intersect with the horizontal, the second time it crosses.

    Small vertical variation on the first intersection, much larger variation on the second.

    Stan

  4. #4
    Need to be low at 30m to be on at 100m

    Sako Ballistics

    Range Velocity Energy Time Trajectory Wind Drift 1 Click Target Lead Range
    [m] [m/s] [J] [s] [cm] [mrad] [click] [cm] [mrad] [click] [cm] [cm] [mrad] [m]
    0 935 2535 0 -4.5 --- --- 0 --- --- --- --- --- 0
    10 926 2486 0.011 -3.5 -3.5 35 0 0 0 0.1 1.1 1.1 10
    20 917 2438 0.022 -2.6 -1.3 13 0 0 0 0.2 2.2 1.1 20
    30 908 2390 0.033 -1.9 -0.6 6 0 0 0 0.3 3.3 1.1 30
    40 899 2343 0.044 -1.2 -0.3 3 0.1 0 0 0.4 4.4 1.1 40
    50 890 2296 0.055 -0.7 -0.1 1 0.1 0 0 0.5 5.5 1.1 50
    60 881 2251 0.066 -0.3 -0.1 1 0.2 0 0 0.6 6.6 1.1 60
    70 872 2206 0.078 -0 -0 0 0.3 0 0 0.7 7.8 1.1 70
    80 863 2161 0.089 0.1 0 -0 0.3 0 0 0.8 8.9 1.1 80
    90 855 2118 0.101 0.1 0 -0 0.4 0 0 0.9 10.1 1.1 90
    100 846 2075 0.112 0 0 -0 0.5 0.1 1 1 11.2 1.1 100
    110 837 2033 0.124 -0.3 -0 0 0.7 0.1 1 1.1 12.4 1.1 110
    120 829 1991 0.136 -0.7 -0.1 1 0.8 0.1 1 1.2 13.6 1.1 120
    130 820 1950 0.148 -1.2 -0.1 1 0.9 0.1 1 1.3 14.8 1.1 130
    140 811 1909 0.161 -1.9 -0.1 1 1.1 0.1 1 1.4 16.1 1.1 140
    150 803 1870 0.173 -2.7 -0.2 2 1.3 0.1 1 1.5 17.3 1.2 150
    160 794 1831 0.186 -3.7 -0.2 2 1.5 0.1 1 1.6 18.6 1.2 160
    170 786 1792 0.198 -4.8 -0.3 3 1.6 0.1 1 1.7 19.8 1.2 170
    180 778 1754 0.211 -6.1 -0.3 3 1.9 0.1 1 1.8 21.1 1.2 180
    190 769 1717 0.224 -7.6 -0.4 4 2.1 0.1 1 1.9 22.4 1.2 190
    200 761 1680 0.237 -9.2 -0.5 5 2.3 0.1 1 2 23.7 1.2 200


  5. #5
    Thanks for the replies folks.

    I do understand the curved nature of the trajectory, and the need to be a bit low at 30.

    But to end up 7 inches high at 100? That seemed extreme.

  6. #6
    The value of a click is different at 30m, did you give it to many clicks ?

    Al

  7. #7
    its not linear

    your 1" high at 30m is climbing significantly

    This is a good site
    ShootersCalculator.com

    this is your scenario in a a chart
    ShootersCalculator.com | Ballistic Trajectory Calculator

    1" at 30m is an effective zero of around 375yds
    the data is a little off but pretty bang on from your observations

  8. #8
    Great - thanks!

    I think, more than anything, this reveals how long you can bunp along THINKING you understand ballistics, when really, you have no clue!

  9. #9
    Quote Originally Posted by Mungo View Post
    Thanks for the replies folks.

    I do understand the curved nature of the trajectory, and the need to be a bit low at 30.

    But to end up 7 inches high at 100? That seemed extreme.
    not really if you are high at 30 witch is the lowest point when it leaves the barrle, is only going to rise up untill it drops off .Like the chart shows .as not to confuse your self just zero at a 100 yards or meters .what ever you like ,and then have alook were your bullet strikes when you shoot it at 30 75 zero 150 then you can learn your trajectory for that bullet ,thats how i would do it start with your zero and work back

  10. #10
    This is my trajectory listing for the same load. My scope height above barrel is 1.75in. It will make a difference if yours is not the same. If different let me know sight height and I'll do a new list for you.

    Click image for larger version. 

Name:	Sako90g243.jpg 
Views:	30 
Size:	117.1 KB 
ID:	43451

    This is theoretical as you will not acheive the Sako velocity for this load and is 935m/s (3067f/s). But its not far out.
    Last edited by billh; 17-06-2014 at 12:35. Reason: Added more info

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