Drop chart dramas

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by gedsar, May 5, 2015.

  1. gedsar

    gedsar Well-Known Member

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    Nov 5, 2007
    Hey guys having a few dramas with my 7mmSAUM.
    I chronoed 160gn accubonds at 2950fps and am using the Isnipe app

    Yesterday I had a shot at a dingo at 400yds and hit under him. I had a second shot at a mark on a tree next to where the dog was and went over and it had hit 11" low. I mucked around with the fps input until I got the correct drop.

    I then had a shot at 800yds and it hit 16" high.

    What do I do to sort this out, or what am I doing wrong?

    Thanks
     
  2. bill123

    bill123 Well-Known Member

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    Jun 14, 2013
    First off, I think that you would end up with a more precise drop chart if you calibrated your program shooting groups at dots on paper from a solid rest instead of single rounds at animals and trees.

    You haven't told us what level of precision that you, your rifle/ ammo combination are regularly capable of. Narrow that variable down first so you know that the error is solely related to the ballistic app.

    I'm not familiar with isnipe but a good ballistic calculator should have a "truing" feature so you don't have to randomly adjust velocity. Maybe isnipe has one and you were using it. I couldn't tell from your post. If it has one and you are certain that the error is not shooter, equipment related, this should be a simple fix.

    Keep us posted
     
  3. J E Custom

    J E Custom Well-Known Member

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    Change the BC of the bullet in the program until it matches the "actual" drop chart.

    Write down as many zeros at different distances that you have, then change the Bullet BC until the program drop matches the actual drop.

    Rarely does the factory BC actually match the actual BC of there bullet perfectly, But it can be adjusted to match your actual drop.

    When you do this the bullet will normally be predictable at all yardages.

    Just a suggestion.

    J E CUSTOM
     
  4. gedsar

    gedsar Well-Known Member

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    OK, Thanks Bill. FYI shooting off a bipod me and my rifle are capable of 5" groups at 600yds. It's def not human or rifle error being 16" out at 800yds. No offence but I'm not that dumb that I can't pick a spot on a white barked tree and measure the drop where it hits, 6 of one half a dozen of other. The pill hit 11" low exactly at 400yds, three shots under my hand. Isnipe was easy to use as it's off my phone. I was chasing info on what people do for their drops specifically when the program doesn't work or if their chrono isnt accurate.
    JE Custom thanks mate is there another app or way to calculate fps I've tried fiddling with BC but it didn't get me close to actual drop.
     
  5. bill123

    bill123 Well-Known Member

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    Since you didn't mention your level of precision and it sounded like you were measuring off of single shots (not groups), it was possible that some of what you were measuring was erroneous information. Glad you got it worked out.
     
  6. J E Custom

    J E Custom Well-Known Member

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    A "Good" Chronograph is essential for accurate velocities. with good velocities and a good drop chart you can get a "Good" Bullet BC. Or with a good BC and a good drop chart you can calculate velocities. You need at least two good components to calculate the third one

    If they don't have access to ether, A drop chart at 200, 300, 600 and 1000 yards using there load Is the best way to make good hits as long as they have a good/accurate range finder.

    With a couple of these tools they can be very accurate, but with out good velocities and drops
    I would not expect to much from a program. (Trash in, Trash out)

    Some times you can get lucky and find a load that is predictable but don't count on it, Prove it.

    Just My Opinion

    J E CUSTOM