# Complete formula

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Hicks, Feb 23, 2011.

1. ### HicksWell-Known Member

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Does anyone know where I could look at a formula that incorporates ALL the variables necessary to calculate a bullet's trajectory? I'm talking wind, spin drift, Coriolis, etc. I'm not interested in lengthy derivations, just would like to see how this can be done by hand.

Also, is there a formula that relates twist rate to bullet stability?

Hicks

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3. ### Michael EicheleWell-Known Member

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If you want to do it easily and quickly by hand without 'lengthy derivations', I suggest you get a hand held computer and use ballistic software. If you really must know, there are good formulas in the Sierra reloading manual 4th addition.

Yes the are formulas that relate to twist rate and bullet stability (see the Miller system below).

([Bullet Weight]*30)/(([Twist Rate]/[Cal])^2*[Cal]^3*[Bullet Length]/[Cal]*(1+([Bullet Length]/[Cal])^2))*([Velocity]/2800)^(.333)*(([Temprature]+460)/(519)*29.92/[Barometric_Pressure])

What you are looking to do with this formula is to apply different twists to come up with a stability factor. 1.0 is concidered barely stable, 1.1 is adequete where 1.35-1.5 is optimum. At least at 1.35 you have some forgivness if you drop major elevation and are in colder temps. A 1.35 could become a 1.2 factor where a 1.1 could become unstable in cold dense enviornments.

Bullet weight in grains, caliber in inches, bullet length in inches, velocity in FPS, Temp in F and barometric pressure in inches of hg.

The greenhill method is easier but not nearly as optimum.

Last edited: Feb 23, 2011
4. ### HicksWell-Known Member

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You know, I have that book, and it is excellent, but I don't see in it where every variable he talks about is tied together in a nice pretty formula. Or maybe a set of formulas. He does some seriously indepth treatments of the different variables though. I'd like to see how all these things fit together.