Effects of bullet spin on drift


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dwwhite
February 11, 2008, 11:12 PM
I've started shooting extensively at 200-300 yards, I've been learning a lot and noticing a few trends. After zeroing to point of aim at 100, then backing off to 300, my point of impact will drift to the right by a couple of inches.

It seems that I read somewhere that the spin of the bullet can actually cause as much as a foot of drift at 1000 yards. Is this a documented phenomenon, or simply something that I am misremembering? Or is it just wind drift (prevailing winds at my range run left to right) that I'm not used to compensating for after years of shooting at closer distances?

If some of you long distance guys could chime in, I'd appreciate it.

edit- for what it's worth, I'm shooting a .308 with 180 grain Match Kings running at an estimated 2550 fps.

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Zak Smith
February 12, 2008, 12:23 AM
You need to use a ballistic calculator that uses the point-mass method to calculate spin drift. With reasonably able LR calibers, you can ignore it to 1000 or a bit beyond. The effect of spin drift is usually less than the error in the wind call, so most shooters just ignore it completely in any case.

The effect you're seeing at 300 vs. 100 is not spin drift.

W.E.G.
February 12, 2008, 12:50 AM
Spin does displace the bullet in the direction of the spin.

Not much at 200-300 yards, but enough you can see it in the scope if you are spotting a shooter at 600+ yards.

In physics, this is called the "magnus effect" or "yaw of repose."

more wiki
http://en.wikipedia.org/wiki/Magnus_effect

Swampy
February 12, 2008, 07:53 AM
The effect of spin drift is usually less than the error in the wind call, so most shooters just ignore it completely in any case.

The effect you're seeing at 300 vs. 100 is not spin drift.

Ditto....

Swampy

Dave P
February 12, 2008, 07:57 AM
my point of impact will drift to the right by a couple of inches.


Sounds like you are seeing effects from the only wind. Back your scope off and look for the mirage walking to the right.

jbm
February 12, 2008, 09:00 AM
The magnus force and yaw of repose are two different issues. The magnus force is smaller (for bullets) than yawing. You need a "modified point mass" to see the effects -- point mass won't do it.

I have one at my website:

http://www.eskimo.com/~jbm/calculations/mpm/mpm.html

At 300 yards, you shouldn't see more than 0.5" of drift (of course it depends on the bullet, wind, spin, etc)

JBM

Art Eatman
February 12, 2008, 10:15 AM
If you run across an old Springfiled '03 with the "ladder" type rear sight, flip it up. Notice that the slot is angled, with the sight-bar moving to the side as you raise it. That was to accomodate spin drift at longer ranges...

Art

Zak Smith
February 12, 2008, 11:10 AM
JBM,

You're the man. I send guys to your ballistic calculator all the time.

-z

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