The Coreolis Effect is brought about by the rotation of the earth and manifests due to the start and finish point of projectile

Coriolis Force No Worry


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Bullet spin drift is generated by the clockwise or anticlockwise spin imparted by rifling.
Bullet spin drift is generated by the clockwise or anticlockwise spin imparted by rifling, ie the bullet moves towards the area of less pressure – in this case to the right.

Q: Can you explain what the Coreolis Acceleration of a projectile is all about? I believe it has something to do with whether the direction of the rifling twist is right-hand or left-hand. I know most modern rifles have a righthand twist.
Geoff Gaynor

A: A projectile fired in the Northern Hemisphere has a Coriolis Acceleration (C.A) drift to the right, while in the Southern Hemisphere it is to the left. The formula is: Earth rotation rate (0.0000729 degrees per second), times double the projectile velocity (fps), times the SIN (latitude), plus for Northern and minus for Southern. Now consider the projectile gyroscopic drift,

The Coreolis Effect is brought about by the rotation of the earth and manifests due to the start and finish point of projectile
The Coreolis Effect is brought about by the rotation of the earth and manifests due to the start and finish point of the bullet on the earth’s surface.

which is to the right with a right-hand rifling twist and left with a left-hand twist. As you can see, if the rifling was given a left-hand twist for Northern latitudes, it would somewhat correct for C.A drift. By the way C.A drift is approximately one inch at 700 yards for a .30-06 projectile. This is critical for accuracy with long-range naval guns such as the 16-inchers on the battleships and is in fact figured into the ballistic computers on board the ships. As far as your average hunting rifle is concerned, just set your sights to compensate for both.


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Nick Harvey

Nick Harvey is one of the world's most experienced and knowledgeable gun writers, a true legend of the business. He has been writing about firearms and hunting for more than 65 years, has published many books and uncounted articles, and has travelled the world to hunt and shoot. His reloading manuals are highly sought after, and his knowledge of the subject is unmatched. He has been Sporting Shooter's Gun Editor for longer than anyone can remember. Nick lives in rural NSW, Australia.

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