True muzzle velocity

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Medicucho, Sep 14, 2010.

  1. Medicucho

    Medicucho Member

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    Hello everybody!does anybody know the equation which permits you to calculate the real muzzle velocity from a recorded velocity in a chronograph which is set at a certain distance from the muzzle? I know that at 10-15ft the difference isn´t significant but I will really thank to know the formula

    Thank you!!!!
     
  2. ss7mm

    ss7mm Writers Guild

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    The info below is from a Google search.
    =============================================

    There is a formula that will closely estimate this loss at normal chronograph distances:

    You will need a scientific calculator to do the calculation. If you don't have one. Window's calculator will work. Click Start ~ All Programs ~ Accessories ~ Calculator to bring it up. Then click the "View" menu and select "Scientific".

    On a scientific calculator, the "e" function is the inversion of "ln" (log number) function and that is activated by pushing "shift" or "inv" + the "ln" key.

    Now:

    Let CD = The Distance, in feet, from the muzzle to the center of the chronograph screens.
    Let BC = The Ballistic Coefficient of the bullet.
    Let CV = The average of the shot chronograph values.
    Let MV = The Muzzle Velocity calculated from the Chronograph Velocity.

    Then

    e( CD / BC / 8816) x CV = MV

    Here's how to do it.

    CD = 7 feet.
    BC = 0.300
    CV = 1500 fps

    On your calculator, enter the following, being sure to enter the parentheses "( )" characters in the equation. The (*) may also be entered as (X) on a hand held calculator and the "=" may be entered as "EXE" or "Calc"

    "shift" + "ln" ( 7 / 0.3 / 8816 ) * 1500 =

    This should return an answer of 1503.975 fps

    Let's do my 280 Remington shooting a Hornady 139 gr. bullet with average chronograph values of 2818 fps

    Chronograph distance is 15 feet.
    Ballistic Coefficient of the Hornady Bullet is 0.392

    e( 15 / 0.392 / 8816 ) x 2818 =

    This returns a Muzzle Velocity of 2830 fps
     

  3. Medicucho

    Medicucho Member

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    Thanks! I´ve seen that formula in a webpage but if you use it and compare the results with the outputs of the JBM page you realize that it is only for G1 model because if you use a G7 model BC the outputs don´t match. Notwithstanding, thanks a lot!
    I´ve found in this link exterior ballistics another probably solution but the equations are too small and I am not able to distinguish the variables very well, please help!
     
  4. SBruce

    SBruce Well-Known Member

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    SS7MM

    I was able to get the same answers you did in your example with my calculator.

    However, when I use the Nightforce Program or the older Sierra Program; the answers derived from the calculator don't always match the printed velocities of either program. I tried a few different bullets and BC's, and the errors or differences between the calculator and the program varied from 0 or dead on to as much as 6 fps.

    Not sure which is absolutely correct.....?? Assuming that the programs us a more complex equation and that they take into account more variables??
     
  5. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    The easiest way to do this is to use a regular ballistics program and set the range step size to one yard. Iterate on the muzzle velocity input until the velocity at 3, 4, or 5 yards (9, 12, or 15 feet) is what you measured with your chronograph. This is much easier than trying to apply a great big long equation.

    -Bryan
     
  6. Medicucho

    Medicucho Member

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    Thanks! that sounds to be the easiest way to do it. Regardless, I would like to know the equation inspite of the fact that it´s too long. I am more or less good at maths and I would like to have a deeper knowledge about it. Thank you for all your contributions , they are very appreciated!
     
  7. ss7mm

    ss7mm Writers Guild

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    Why don't you just contact Sierra and ask them if they'd give you the formula as you can't read what's on their site. Since they have posted the formula on the web I don't see why they wouldn't give you the formula in a format that you can read.
     
  8. Hector Rodriguez

    Hector Rodriguez New Member

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    I just use Aardvark Muzzle Velocity app on my smart phone to calculate the true muzzle velocity.
     
  9. pyroducksx3

    pyroducksx3 Well-Known Member

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    Chronos lie! It doesn't really matter if it's 15 ft away all the chrono does is get you close enough to use your ballistic program. Once you start shooting out you will be able to adjust your fps to match the data you are collecting. So in my opinion trying to figure this out is a waste of time.
     
  10. J E Custom

    J E Custom Well-Known Member

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    If you have a chronograph that has a proof screen it will tell you the velocity loss between screens
    and with the known distance from the muzzle you can calculate the muzzle velocity based on the loss in velocity in 2 feet increments. (It wont be exact, but it will be close).

    One other method is to buy a Magneto Speed. (It measures velocity @ the muzzle).

    When all else fails and/or you don't have a chronograph,you can shoot at least 3 rounds at 100,200,300,400 and 500 or 600 and measure drop at the center of all groups and compare it to a drop chart that has velocity and trajectory numbers. In any case this should be done to assure that your data(velocity, BCs Of bullet, and MOA are all correct before taking a very long shot.

    Not very scientific, But they work.

    J E CUSTOM
     
  11. Mikecr

    Mikecr Well-Known Member

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    I don't think this is true.
    To begin, timing between any screens is an average of velocities to cause that timing. That is velocity passing the 'start' screen is not actually the velocity passing the 'stop' screen.
    And I believe the 'proof' screen is still taken to the same 'start' screen, and integrated at twice the distance.
    That said, I'm pretty sure my Oehler velocity difference between primary & proof channels represents the inescapable averaging of passing velocities(causing error, and printing as different).

    The proof channel is only to validate the primary channel, and not showing velocity at one distance versus another distance.