 True muzzle velocity 
09142010, 11:19 AM

Junior Member


Join Date: Sep 2009
Posts: 9


True muzzle velocity
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 1015ft the difference isnīt significant but I will really thank to know the formula
Thank you!!!!

09142010, 11:39 AM

Platinum Member


Join Date: Jun 2005
Location: Yakima, Washington
Posts: 3,833


Re: True muzzle velocity
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
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ss7mm

09142010, 01:14 PM

Junior Member


Join Date: Sep 2009
Posts: 9


Re: True muzzle velocity
Thanks! Iīve 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 donīt match. Notwithstanding, thanks a lot!
Iīve 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!

09142010, 03:09 PM

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Join Date: Oct 2009
Location: Thunder Basin, WY
Posts: 1,788


Re: True muzzle velocity
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??

09142010, 03:19 PM

SPONSOR


Join Date: Mar 2007
Posts: 511


Re: True muzzle velocity
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

09152010, 07:38 AM

Junior Member


Join Date: Sep 2009
Posts: 9


Re: True muzzle velocity
Thanks! that sounds to be the easiest way to do it. Regardless, I would like to know the equation inspite of the fact that itīs 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!

09152010, 08:42 AM

Platinum Member


Join Date: Jun 2005
Location: Yakima, Washington
Posts: 3,833


Re: True muzzle velocity
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.
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