It invariably always pops up in the vernacular of people in person or on the Internet who try to sound edumacated about gun ownership – and those who pretend to be something they aren’t.  The “Coriolis Effect”, they say, must be taken into account at long range shooting or you can’t hit the broad side of a barn.

Photo via Gizmodo.

I’ve personally heard a young gentleman trying to impress some novices at a gun shop by telling them how he accounts for the “Coriolis Effect” in shots as close as 300 yards!

I was impressed.

Sure you do, buddy.

It turns out that the Eötvös & Coriolis Effects are indeed factors in long-range ballistics.   Miniscule factors, compared to wind changes, air currents, temperature fluctuations, altitude, etc.

First off, one should know that the Eötvös effect is a vertical component based upon the centrifugal force of the earth.  In short, if you fire towards the east, the bullet will resist gravity in minute amounts.  Firing towards the west, it’s just the opposite.  It’s most significant at the Equator and null at the poles.

How minute is it?  Typically less than 1%.  In plain English:  If your bullet drop is 400 inches at 1,000 yards, the Eötvös Effect will be +/- less than 4 inches.

The Coriolis Effect is a horizontal component, of a similar magnitude.  It’s greatest at the poles, of course, and null at the Equator.  You knew that, right?  You probably also knew that it deflects to the right in the northern hemisphere and the left below the Equator, right?

The effect of a 2 mile per hour wind change would be far more dramatic than the Coriolis Effect.

Here’s a more technical description of these effects so the next time you hear a pretend expert in long-range marksmanship trying to dazzle some newbies with B.S., you can know for yourself just how clueless they are.

…When talking about ballistics, the Coriolis Effect refers to the deflection on the trajectory of the bullet generated by the spinning motion of the Earth. Its effect is negligible at medium distances, but becomes important around 1000yds and beyond, especially because it can add to other minimal errors and keep you off target.

Coriolis effect affects everything not firmly attached to the Earth’s surface. It affects fluids, like air and water, as well as floating and flying objects like ships, airplanes and… bullets.

Despite being associated with Coriolis, the phenomenon that actually affect the vertical component of the trajectory is called Eötvös Effect. The rotation of the Earth generates a centrifugal force, the same that pushes you to the side when you make a sharp turn with your car. This force act perpendicular to the Earth rotatory axis, adding or subtracting to the gravity force. When an object flies eastward, in the same direction of Earth’s rotation, centrifugal force acts opposite of gravity, pushing it away from the Earth’s surface. If the object flies westward, in the opposite direction of the Earth rotation, centrifugal force pushes the object toward the ground concurrently to gravity force. Thus, bullets fired to the east always fly a little higher, and, conversely, bullets fired to the west always travel somewhat low.


How high or low?

Here’s an example of error due to Coriolis effect: firing the same .308 175gr bullet at 2700fps muzzle velocity, from a latitude of 45° in the Northern Hemisphere, the deflection at 1000yds will be of 3in to right. At the North Pole, where the effect is maximum, the deflection will be a little more than four inches. The deflection will be the same in the Southern Hemisphere, but it will be to the left, instead.


I’ve yet to meet a man who could hit a 10″ plate at 1000 yards.

The odds of you meeting one of these shooters who perform so well with such precision equipment talking up his exploits at your local gun shop is somewhere between slim and extra slim, if you know what I mean.  So keep that in the back of your mind the next time some Special Snowflake is trying to dazzle someone who doesn’t know better.

Like W.C. Fields once said, “If you can’t dazzle them with brilliance, baffle them with bullsh*t.”