Need help Calculating how high to sight at 100 to hit at 500

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by thorbird, Jul 22, 2010.

  1. thorbird

    thorbird Well-Known Member

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    Can anybody with the Sierra ballistics program, or any program, help me out and tell me how many inches high I need to be at 100 yards to be on at 500?

    I have 2 guns I need to know this for:

    The first is a 6BRX with the Berger 108 BT (.243) traveling at 3050 FPS.

    The second is a .308 with the 168 SMK (.308) traveling at 2,700 FPS.

    The following is the same for both: They are both zeroed at 100 yards. The altitude would be 2,000 feet above sea level and temps at 75 degrees.

    I can’t find my Sierra program and I need this information by Saturday so I don’t waste a bunch of ammo getting on target. My guess is that I probably need to be about 9 inches high at 100 for the 6BRX, and about 11 for the .308?

    Thanks so much to anyone who can help me out.
     
  2. bigngreen

    bigngreen Well-Known Member

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    I'll take a stab, 6mm is 7.54 MOA and 10.65 MOA for the 308, using Bergers ballistic program.
     

  3. NomadPilot

    NomadPilot Well-Known Member

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    Go here. JBM - Calculations - Trajectory

    Fill it out and set the "Zero Range" to 500. Make sure "Elevation Correction for Zero Range" down at the bottom is checked. Click Calculate, then look at the Drop (in) column for your "inches high" at 100.
     
  4. Eaglet

    Eaglet Well-Known Member

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    What's your scope height?
     
  5. johnnyk

    johnnyk Well-Known Member

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    I used JBM and see the 6BRX as 9.0" high and the .308 at 12.5" high, at 100yds. The only thing I "assumed" was the 1.5" scope height. I use JBM exclusively and when dialed in for 500yds my rifles are usually within an inch or two of being dead on, elevation wise. The windage is a beast unto itself. I usually go with the standard 10MPH crosswind and make an adjustsment after the first shot. I believe this will get you on paper at 500yds. Good shooting! JohnnyK.
     
  6. thorbird

    thorbird Well-Known Member

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    Wow. I was hoping someone would respond, and here I got all this great info including learning about JBM.

    Thanks a bunch.
     
  7. bigngreen

    bigngreen Well-Known Member

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    JBM is a good tool, I use it to check my field programs against. I've been using the one on the Berger web site lately to see how it works, it is way simple!
     
  8. thorbird

    thorbird Well-Known Member

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    Thanks for the info on the Berger program. I'll check that out too.
     
  9. LouBoyd

    LouBoyd Well-Known Member

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    It's not good to use the default value or assume the hight of the sight above the bore. It can be under 1" for some metallic sights over 2" for some ARs, and much higher for some night vision devices. It's easy enough to measure for a particular rifle. I try to get the value correct to 0.01" Technically it's the hight of the centerline of the optical path to the target center over the center of the bore at the rifles crown, , but for most practical purposes it can be measured at the front of the scope relative to the barrel. For good accuracy measure from the top of the bell of the scope to the underside of the barrel (action removed from the stock) with calipers. Then measure from the underside of the objective bell to the top of the barrel. Add those numbers and divide by two.

    It will drive you nuts trying to get a rifle to match zeros with a ballistics program if that number is not entered correctly. The error is multiplied by the ratio of the rifles zero distance to the downrange distance and screws up all subsequent drop calculations.
     
  10. topbrass

    topbrass Well-Known Member

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    I just measure the dia of the scope and the dia of the barrel below the scope measurement and add them together and divide by 1/2. Then add the distance from the scope to the barrel to get center of bore to center of scope. No need to remove the stock, unless it is something like a AR.
     
  11. thorbird

    thorbird Well-Known Member

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    Thanks for all the great info. I was right on at 500, but decided to just shoot the 6BRX. This was a fun local get together where you shoot 3 shot groups. Person with smallest group wins.

    My 3 shot groups at 500 were:

    1 7/8
    1 5/8
    1 3/4
    1 5/8
    2 1/4

    I was happy. I shot the smallest average of the about 80 shooters that day. But the prize went to a 7/8 inch group.