Home Forums Hunting & Shooting Hunting Rifles Bullet spin causes float?

Spin drift can amount to as much as a full Minute at 1000yds with a 308 target rifle. Comparatively minor considering wind etc. but there nonetheless and some of the best long range shooters do account for it........DJ

Vaugh in his "Rifle Accuracy Facts" gave the first explaination I ever read of why "Wind Drift" is a function of lost velocity
(delay time if you will), and not time of flight.
As the bullet is pointed into the wind (zero angle of attack) part of the drag is a down wind vector. The drag (B.C.) is defined in terms of velocity loss. That is why bulllets drift down
wind. So much B.S. has been writen on the subject. The next
most screwed up concept is the effects of shooting up hill, or down.

Good Luck!

CAS I'll admit it's a little highbrow for the everyday shooter but the benchrest shooters have been observing this for years.Considering the fact that a bullet in flight is rotating about 200,000 rpm a crosswind has an effect on the point of impact.Not only are the shooters watching the direction of their wind flags they are watching their speed also and adjust their hold accordinly.The rotating bullet "grabs" the air and will go high or low depending on the crosswind.The wind flags aren't just used to judge windage but elevation as well.A right twist barrel shoots differently than a left twist barrel.We're only talking about thousanths of an inch here but it is true and it's what wins matches. Truly an art form in itself.

As for me. I'm just happy as a clam if I hit the deer <img src="/ubbthreads/images/graemlins/laugh.gif" alt="" />

284LUVR is exactly right. A rh twist barrel shot in a 3:00 wind will hit higher than a LH twist barrel shot in the same wind. Were the wind to switch to 9:00, the results would reverse. Somewhere in my BR files I have a chart that shows how much the effect is, depending on the angle of the wind.

It is important to clarify, though, that this effect does not cause the bullet to hit "high", just "less low". The bullet is still dropping from the muzzle onward--this effect is nowhere near significant enough to alter the fundamental trajectory. But such things matter in the BR game, where a five-shot five group aggregate of .1800" might win a big match, a .2300" might put you middle of the pack and a .3000" would have you shopping for a new barrel or thinking about golf.

I'll admit to not being as knowledgable about ballistics as many of you. After thinkin' about this a bit, I can believe the rifling marks on a bullet can act like the dimples on a golf ball or stitching on a baseball to produce " bullet spin drift" by way of pressure differential.

Now I have a new explanation for my groups! <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />

--Mike

It's more than that at 200 yards. See the attachment to my previous post.... 1" at 200 yards, in a 20 mph crosswind.

You ought to get one effect while the bullet is climbing, and the opposite effect when it is on the other side of the peak. So there ought to be some way to get your bullet to peak in the right place, so that the effects net out to zero.....

I'm fascinated by the things I find in Harold Vaughn's book. I'm just disappointed that he departed the planet before I could get to know him.

It's the same phenomenon that causes "P" factor in single engine prop planes. It has to be compensted for.

Twin engines spin in opposite directions to cancel out the effect.

Of course, were talking on a much smaller scale, and it's not worth mentioning at the couple hundred yards most of us shoot in. If you were shooting for miles - as in heavy artillery - it has to be taken into consideration.

Denton; I had the honor of talking with Vaughn.His book was so exciting to me that I looked up his tel#. He sounded weak
but was very gracious. He was a real man, and will not be replaced soon. His book should be read by anyone that wonders "what is really going on". So many intuitive answers are wrong,and repeated by the "experts".
Good luck!

The doubters amongst us should take heed to what dj said for he is correct on this subject.

As to the original question, the "sine wave" has nothing to do with "spin drift", but it does exist in virtually all bullets because of gyroscopic precession and the deflections associated with it. In truth it is more of helical path, 3 dimensions being involved. Deep subject, I'm not going to write a book on it here.

Crosswinds cause aerodynamic jump(see Vaughn or McCoy), imbalance in the bullet cases ballistic jump(same). Spin Drift should not be confused with Wind Drift as they are distinct as separate issues. Too, they are not "drift" but a deflection.

You may not want to say a bullet is an airfoil but all rotating cylinders generate lift when subject to a wind component transverse to the axis of rotation. A fired bullet and dropped bullet will NOT strike the ground at precisely the same time for a different reason however, that being the fact that the axis of rotation lags slightly behind the bullets path, thus generating a slight positive angle of attack from an aerodynamic standpoint. Google "Tractability" if you're bored, or see this: http://www.nennstiel-ruprecht.de/bullfly/index.htm#Formulas.

You fellas got a thread going here that real rocket scientists spend their lives on, it won't give you the education in a 10 minute read. Either spend some time on the research or believe the earth is really flat, it's up to you.

DDan, Good link. And thanks Dixie, a good discussion like these two is one of the few things that can make me dig up the old engineering physics books. A little mental exercise is a good thing.................DJ