Project Appleseed

Does anyone have a formula for "spin drift".
Thanks, Cowboy

By far I know little when it comes to things of importance.  I can, however, claim to recognize topics or answers when presented with them.  Spin Drift is a topic I truly have never heard of, however.  I google'd it once I stumbled on this post and now am very interested to see what the experts here have to say.  Maybe I'll get my Physics book out and see if I can find anything on the subject.

I'm certainly not a long range expert, but I can give a target at 400 yards a very hard time. 

My non-expert advice is not to sweat the small stuff. Wind, rifle accuracy and shooter ability will all be greater variables than spin drift. Heck, bullet voids and Standard Deviation are probably greater issues.

http://en.wikipedia.org/wiki/External_ballistics#Gyroscopic_drift_.28Spin_drift.29

Good discussion down the page.  No formula, you will see why if you read it.

SoM

Spin drift can be ignored in actual shooting. When zeroing a rifle, it is part of and included in, the zeroing process.

Now for actual long range shooting (1000 yards) the actual spin of the earth needs to be compensated for when shooting north (Camp Perry) or south Camp Atterbury. One 1/4 minute click is enough. But again is part of the zeroing process so don't get a headache over it...

Quote from: jmdavis on March 24, 2010, 09:24:42 PM
... Standard Deviation [is] probably [a] greater issues.

Hehe, yeah, bingo! I definitely have an issue with standard deviation --- mine's too big!

Quote from: Son of Martha on March 24, 2010, 10:09:39 PM
http://en.wikipedia.org/wiki/External_ballistics#Gyroscopic_drift_.28Spin_drift.29

Good discussion down the page.  No formula, you will see why if you read it.

SoM

While my marksmanship is nowhere near the point that I need to worry about this "spin drift", the Wikipedia read is indeed interesting. The math is a bit fuzzy, but interesting. 

"For right-handed (clockwise) spin bullets, the bullet's axis of symmetry points to the right and a little bit upward with respect to the direction of the velocity vector as the projectile rotates through its ballistic arc on a long range trajectory." We've all seen this when watching football on TV, when the camera captures excellent footage of a long pass, from an angle more or less in line with the pass. I've wondered what caused it.

"As an effect of this small inclination, there is a continuous air stream, which tends to deflect the bullet to the right." Cool.

The Wiki page also has a table indicating the magnitude of this effect for various projectiles. According to that, at 1000 yards, a 55-grain .556 bullet can experience a 23 inch drift due to this effect! Maybe someday my marksmanship will be good enough that I have to worry about this.

-FM.

Fuzzymath, I too wondered why a football or bullet would stay point-on when it went through an arc starting with an upward angle and curving to a downward angle.

After much head scratching I decided that it had to be air pressure on the underside of the projectile. If you think about a bullet in flight at an upward angle, if it were to then begin falling, but staying point-upwards, the flank of the bullet on the underside would be exposed at this angle to the air stream, while the flank on the opposite, upward side would not, and in fact would be somewhat hidden behind the up-tilted point and protected.

This upward-pointed angle with a downward-plunging bullet, would produce a relative high air pressure on the exposed downward flank of the bullet, and a relative low air pressure on upward-facing flank. This disparity of pressure will result in the bullet being pushed straight into a 'neutral' position of least resistance, which is point-on in the direction of the bullet's motion. If this seems unlikely, remember that it is similar in effect to the differential air pressure over an airplane's wing which causes lift. And a 747 lifting off the ground isn't going near as fast as a bullet.

Take a look at an 03 Springfield to see how the ladder tapers to the left as you raise the sight up.

I don't fuss with it, but JBM Ballistics has a web  page with every conceivable ballistic correction/calculation, including spin drift.  For more, visit
http://www.jbmballistics.com/calculations/calculations.shtml   There's plenty of free stuff, enough to boggle your mind on side issues that don't really matter, but are nice to know if they're of interest to you. 

In small bore silhouette many shooters add windage of 1/2min at 77meters and 1 minute at 100 meters with target .22lr.
knowing which direction you rifleing turns would be vital.
Not adding windage corrections is still within the 4 minute standard.

For my .300 RUM w/ 220 gr SMK, the spin drift is about .5 moa at 1000yd.  It is less at closer range; .2 moa at 500.  The reason it increases with range is that the rpm's don't decrease as much as velocity does, i.e., the bullet is still spinning like crazy at 1000yds, but only going about 1800 fps instead of about 3000.
-SG

There is also the fact that the bullet started its flight in a slightly nose up orientation, do to the fact that it starts under the line of sight, crosses the line of sight somewhere near 82' from the muzzle, arcs to its highest piont until gravity overcomes it upward inerta.
From that point untill impact the bullet is accelerating down while the spin of the bullet keeps it's nose up it's entire flight.
As the bullet's path curves downward it has less of it's point breaking the static air pressure and more of the spinning side of the bullet bearing on the air it is flying through.
My way of thinking is that is where the side slip begins, where the bottem side of the bullet has more traction on the air than the top side of the bullet.
That arc and down fall begin to affect a .22 long rifle much closer to the fireing line than long sleek bullets used by most long range shooters.

Old Dog nailed it.
Spin drift is something that sounds cool to talk about.
For 99 percent of the shooters out there, make that 99.9 % it is a non-issue.
Forget "spindrift" and get out there on the line and practice the basics !!!
My .02