Drop charts

Discussion in 'Long Range Hunting & Shooting' started by squirrelduster, Jan 3, 2013.

  1. squirrelduster

    squirrelduster Well-Known Member

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    Had a strange experience at the range today.
    Working up drop charts for my 6.5x.284. Berger 140 gr. 2930 fps through them chrono during load work up, no chrono today.
    Shot a golf ball sized rock at 100 yards to verify zero.
    Target at 500 yards.
    Dialed in 8.5 minutes from my iPhone ballistics app.
    Shot high.
    Dialed down to 7.5 MOA
    Shot high
    Dialed down to 6.5 MOA
    Perfect elevation. 3 inches to the left due to wind. Compensated and the next two were 1/2" apart. My deer rifle really shoots.
    My question is, to get the drops correct the velocity needs to be 3250 fps. I have never had to change my velocity this much.
    Anybody else seen this?
     
  2. dragman

    dragman Well-Known Member

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    I have ran into the same thing. I chonod my 300 rum @3200 for my program to hit where i have verified its hitting my adjusted speed is 3385.
     

  3. cornchuck

    cornchuck Well-Known Member

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    I have come to learn not to pay any attention to what the chrono says for velocity. I have been using it to get ES and SD. But everyone has their own reasons for using a chrono. Which is fine with me.

    Jason
     
  4. cwinner

    cwinner Well-Known Member

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    I too have seen this from time to time, one thing to check is how your scope is tracking.... Set up a 100yd target, now use a level to extend a perfectly strait line upwards of 30".......put a 1" target at 10", 20", and 30"....shoot the 100 yd zero, dial your scope 10MOA, shoot another ( hopefully you hit the dot 10" above) then repeat for the 20&30 MOA and see where it shoots......i have one scope that tracks 5% off meaning if I dial 10 MOA (40 clicks) I'm high on the 10MOA, if I dial 38 clicks I'm dead nuts......next take this and enter it in as your elevation correction factor in shooter and it will help......forget the .06 MOA....you won't notice .06" at must normal ranges.
     
  5. WildRose

    WildRose Well-Known Member

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    What direction was the wind coming from and was it level, rising, or falling ?

    If your scope is dead on you might want to try adjusting your BC. Remeber BC's are not static, as velocity drops off so do BC's. The BC's we get from the factory specs are essentially averages based on what ever ranges they are testing the bullets at or based on what a computer program spits out.

    Your Chrony, Scope, Rifle may all be dead on but the BC a bit low for the velocity you are running at the muzzle.
     
  6. squirrelduster

    squirrelduster Well-Known Member

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    Winner, the scope is a Nightforce that has been on multiple rifles with no issues before so I doubt if it has anything to do with the problem.

    I am planning on going out again next week with the adjusted velocity and checking different ranges to see if it holds.

    I adjusted the bc up to .700 and it didn't make much difference at such a short range.
     
  7. WildRose

    WildRose Well-Known Member

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    The difference at long range should be more pronounced.

    I'd keep tweaking both the BC and velocity until you actual drops at 3 different ranges match up.

    I like to use 200, 500, 700yds. If you can get it pretty close at three different ranges then your chart should be pretty well true at any range.
     
  8. Greyfox

    Greyfox Well-Known Member

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    Being a 6.5x284 shooter, it is very unlikely that 6.5 MOA is the actual drop for 500 yards with a 100 yard zero. Your velocity of 3250FPS is not right. Shooting a golf ball sized rock for establishing your 100 yard zero is not precise enough for accurately determining drop data out at 500 yards. This is very likely the compounded variation you are experiencing at 500 yards. Having used numerous lots of 140 VLD's it also not likely that the BC of your bullet is that far off the published spec. I don't trust chrongraphs that much but your chronographed velocity of 2930 sounds realistic and even a variation of 50 FPS wouldn't give you the drop variation you are experiencing at 500 yards. I determine the drop data for my 6.5 x284 using a 200 yard zero that has to be nuts on with at least a 1" perfectly centered group. At 100 yards, .5 MOA. The ballistic data/drops, assuming all other variables are entered correctly, using the Bergers are always very close to expectation. IMHO.
     
  9. squirrelduster

    squirrelduster Well-Known Member

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    I definitely have more work to do.
    The rock shooting was just to make sure the zero didn't move.
    I need a trip to a real range that I can shoot to 600 yards. Closest one is 3.5 hrs away.
    Probably head to some BLM land next week to rezero at 100 and record drops at 300, 400, and 500. Want to be able to shoot to 1k with accuracy. Have a great load worked up so I need to get my drops correct.
    Thanks for all the help.
    I will repost with my findings.
     
  10. cwinner

    cwinner Well-Known Member

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    You never know until you test ............believe only what you can verify. If its a nightforce, then nothing's every going to possibly be wrong with it.

    This is a trick I learned from a SF guy, the first thing they did with any new equipment set up ( mostly S&B scopes BTW).

    Shoot four three shot groups and Prove me wrong, for your own sake! It will only take 12 rounds, good luck with your range session.