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Old July 7, 2013, 11:00 PM   #6
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,811
'Twas either Hatcher or Townsend that proved decades ago that the .30-06 with 172-gr. FMJBT bullets have about 6 to 7 inches of spin drift at 1000 yards. There's not much gyroscopic precession that causes spin drift when the angle of bullet departure to hit at 1000 yards is about .75 degrees and the bullet's angle of fall at the target is about 1.7 degrees; the bullet tilts only 2.45 degrees during flight from its angle of departure. It won't precess very much going that far.

Which means if you sight in at 1000 yards and get a zero with bullets striking a couple inches to the right, they'll be right on in windage at about 700 yards, an inch or more to the left back through about 400 yards, then a lot less to the left back to the muzzle.

But spin drift does happen; there's web sites with calculators to figure it out. If you shoot your stuff into no worse than 3/8 MOA through 1000 yards, then yes, worry about spin drift. Best wishes reading the wind past the first 100 yards and making exact corrections for it all the way to 1000 yards, too, as the wind drift for each yard of bullet travel's a lot more the closer the bullet gets to the target. You can only measure wind speed where you are at; good luck estimating crosswind speeds at various points downrange to the target. The wind speed's faster at the highest point in the bullets trajectory above the line of sight than at the muzzle and target; how much depends on the terrain. And a 1 mph crosswind moves .30-06/.308 bullets about 10 inches at 1000 yards.

I've never changed windage zeros for a .308 Win. or 30 caliber magnums from 100 through 1000 yards. And never ever heard folks at a match getting concerned about spin drift. The subtle cross winds that are always out there mask any drift from bullet spin for all practical purposes. Besides, spin drift's the same for every shot fired at a given range. It's the trajectory in the horizontal plane.

Fewer than 10% of the best long range, top ranked competitive shooters on this planet will put their first shot down range where they've shot before, got good zeros for their ammo there, and have it strike within 5 or 6 inches of their point of aim on a target 1000 yards away.

Therefore, in looking at the realities of spin drift and its relationship to all the other things that move bullets off their desired trajectory, I consider it negligible.
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Last edited by Bart B.; July 8, 2013 at 07:16 AM.
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