HappyHellfire
Silver $$ Contributor
Is spin drift important for your long range shooting? How do you know how important spin drift is to say Coriolis or cross wind? I'm going to try to explain what spin drift is and how to estimate it.
Bullets do not fly perfectly straight. Even the best, most stable bullet will have what we call epicyclic motion or bullet wobble. The nose of the bullet wobbles up and down (pitch) and left and right (yaw) as it flies. We have the math to study this motion, its called Linear Projectile Theory. If we solve for a spin stabilized bullet fired horizontal, we get a pretty little pattern.
This is for a 168 grain Sierra fired at a Mach 2.4 (2700 fps) from a 1:10" barrel. The center of this shape is slightly offset to the right for right hand rifling and produces a tiny average force that pushes the bullet to the right. With a good bit of math we can estimate how much the spin drift is by the equation:
This is one of the only good reasons not to over-spin a bullet. If the bullet is stable at a 1:10 twist and you fire the same bullet out of a 1:8 twist, the increase spin (p in the equation) will increase the spin drift. For a well designed rifle and bullet combo, the spin drift will only be about half of the Coriolis drift and the Wind drift will dominate the cross range motion of the bullet.
If you like the math, I have a link to my book on external ballistics on my website https://www.aerospikebullets.com/the-bullet-guy . Don't buy it unless you have a good bit of advanced calculus under your belt, it will be a waste of money but if you are weird like me you might find it interesting.
Bullets do not fly perfectly straight. Even the best, most stable bullet will have what we call epicyclic motion or bullet wobble. The nose of the bullet wobbles up and down (pitch) and left and right (yaw) as it flies. We have the math to study this motion, its called Linear Projectile Theory. If we solve for a spin stabilized bullet fired horizontal, we get a pretty little pattern.
This is for a 168 grain Sierra fired at a Mach 2.4 (2700 fps) from a 1:10" barrel. The center of this shape is slightly offset to the right for right hand rifling and produces a tiny average force that pushes the bullet to the right. With a good bit of math we can estimate how much the spin drift is by the equation:
This is one of the only good reasons not to over-spin a bullet. If the bullet is stable at a 1:10 twist and you fire the same bullet out of a 1:8 twist, the increase spin (p in the equation) will increase the spin drift. For a well designed rifle and bullet combo, the spin drift will only be about half of the Coriolis drift and the Wind drift will dominate the cross range motion of the bullet.
If you like the math, I have a link to my book on external ballistics on my website https://www.aerospikebullets.com/the-bullet-guy . Don't buy it unless you have a good bit of advanced calculus under your belt, it will be a waste of money but if you are weird like me you might find it interesting.
