Problem with accurate readings from ballistics progs.

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Savagebien, Dec 5, 2008.

  1. Savagebien

    Savagebien Well-Known Member

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    Hey, i shoot a savage in .243 and i keep having problems getting JBM and an excel program called pejsa ballistics which is in excel to give me accurate predictions with the real weather conditions, and bullet speed and data. For example, i can get JBM to give me everything correctly with the real temp, humidity, and elevation, real bullet data, which is 6mm 105 Amax, and 100y zero which i use when shooting, but i have to insert 2960 ft/s at the muzzle and i chrono'ed this load at 2800. Now using pejsa ballistics with the same bullet info, REAL velocity of 2800, scope height set to 2.75 in when its really more like 1.75, i measured, and the temp set to 75 celsius which is WAY hot, i can get it to b about spot on. y is this? just a wierd occurence? something to expect from free ballistics calculators?
    Thanks for reading and any advice!
     
  2. MagMan

    MagMan Well-Known Member

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    Leave your conditions and velocity as they should be and modify your BC to make things right.
     

  3. Savagebien

    Savagebien Well-Known Member

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    thanks magman, i tried that, ended up using .76 for bc....... and its within .2 or so MOA up to 550y, being slightly over. and then gets up to .75 moa off at 750y. then 1.25 off at 850y, giving less drop than real world, so i guess it works well enough. thanks again!
     
  4. jwp475

    jwp475 Well-Known Member

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    IME it culprit is an inaccurate chrono reading .

    Example, I originally chronographed my 338 Lapua with the 300 SMK and came up with 2820 FPS average velocity. Actual shooting revealed that my drops were 6.4 inches low at 975 yards. Next I ran the BC up to .830 (.768 is Sierra's listed BC) and this did not solve my problem
    Next I decided to recheck my zero and to re-chrono my load at the same time and this time I came up with 2791 FPS average velocity. Using 2791 FPS my drops were spot on and I have not experienced another problem
    I was using an Olher 35 the difference was the lighting one day was very bright and sunny and the other day was overcast. The overcast readings were more accurate. That was an error of only 29 FPS and that was the problem

    ExBall will allow you to trajectory validate the program and this adjusts velocity not BC
     
  5. Savagebien

    Savagebien Well-Known Member

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    thanks JWP475, i pm'ed someone on here i know asking them to check exbal with my data to see if its more accurate. i chronoed on a sunny day at 2800 avg. and then last weekend i shot the same load but it was about 35-40 deg instead of 75 and it dropped to about 2680 ft/s b/c of the powder's temp sensitivity i suppose. and it was cloudy last weekend when it was colder too. so idk i'll keep trying stuff, thanks!
     
  6. MagMan

    MagMan Well-Known Member

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    jwp, I'll agree with that....makes sense to me and a bad reading is probably more common than one would think.

    The last chrono I had previous to my M2 would read my 338 Edge at 2956 fps with 92 grains of H1000 & a 300 SMK. I knew that thing was wrong....and there was no doubt.:D
     
  7. Savagebien

    Savagebien Well-Known Member

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    ok guys, i just altered the scope height, set it to 2.75in, and then changed the bc to .55 and its pretty dang accurate, i think i can live with it like that for now! thanks for the advice