Sunday, March 6, 2011

Spin Drift and Coriolis Effect

This post is a continuation of Long Range Shooting (with style).

How to Calculate Spin Drift and Coriolis Effect
Isn't it great when you take the 1000 yard shot with no wind only to find that you are off by 3 minutes to the right,  and no Idea why.  Now you will have the information to explain why.  There are two factors that will affect your bullets flight no matter the weather.  I can't pretend that I am the expert on this topic, but I don't think anyone on earth really is (call David Beckham he might know).

Spin Drift
Spin drift is a linear drift of your bullet caused by the rotating motion of your barrels twist.  If you have a barrel that spins clockwise then your bullet will drift right and if it spins counter clockwise then it will drift left.  Most barrels are clockwise.  If you are interested in the exact physics then I will have a link at the bottom of the page that will go into great detail.  In most cases your bullet will drift 6-10 inches at 1000 yards depending on your velocity, BC, barrel twist and probably ten other factors I don't know about.  JBM Ballistics has a free online ballistics calculator that accounts for spin drift.

Coriolis Effect
This one is great this is drift caused by the rotation of the earth.  In my opinion this won't make a difference for anyone who doesn't expect to make a cold bore shot at 1500 yards.  whether it makes a difference or not here are the facts.  Coriolis drift completely depends on your longitude and the direction you are shooting.  If you are shooting north or south your drift will be to the right if you are in the northern hemisphere and left if you are in the southern hemisphere.  If you are shooting east or west then it will affect your drop.  Shooting west will make your bullet drop more and shooting east will make your bullet drop less.  At 1000 yards you may be talking about 1/4 MOA.

I went to a web page that had a equation that gave the specifics of the Coriolis effect.
Here is the gravity multiplying equation for the Coriolis effect (for excel) 

A1 = muzzle velocity
A2 = latitude
A3 = Azimuth (compass reading)

The end result should be multiplied by the drop of your bullet.

Here is the horizontal drift equation for Coriolis Effect (for excel)

C5 = range in feet
B6 = latitude
B7 = flight time of the bullet


The end result is in feet, so multiply by 12 to get inches of drift (right in the northern hemisphere)

as always shoot a lot and always write it in your data book and study the results of your shooting.  Track the patterns and you will be able to make shots you never dreamed of.

"Nothing can stop the man with the right mental attitude from achieving his goal; nothing on earth can help the man with the wrong mental attitude." ~ Thomas Jefferson

return to Long Range Shooting (with style)


  1. Hi Andy!
    I could not find the link you mentioned that would "go into more detail".
    Thank you!

  2. This comment has been removed by the author.

  3. Couldn't the horizontal drift be written as


    Am I missing something here?

    I, also, can't find the "link to more detailed explanation" document.

  4. It doesn't matter which hemisphere you are in. Coriolus always goes to the west.