Wind and Spindrift

Litehiker

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Today I was at our club's steel silhouetta range shooting out to 960 yards. We had about a 10 -12 mph wind from 2 o'clock.

I was on my shooting mat with Atlas bipod and CTK monopod which I have found is as accurate as shooting of a cast iron rest and a ear bag.

So I corrected about 1 mil right and FORGOT about spin drift. My impact was to the right, off the 10" gong by about 6 inches.

Hmmmm... I shoulda corrected less and let the right hand twist spin drift do most of my work for me.
This is a reminder not to forget about spin drift and know the direction of your rifle's twist.
 

Dog Rocket

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Today I was at our club's steel silhouetta range shooting out to 960 yards. We had about a 10 -12 mph wind from 2 o'clock.

I was on my shooting mat with Atlas bipod and CTK monopod which I have found is as accurate as shooting of a cast iron rest and a ear bag.

So I corrected about 1 mil right and FORGOT about spin drift. My impact was to the right, off the 10" gong by about 6 inches.

Hmmmm... I shoulda corrected less and let the right hand twist spin drift do most of my work for me.
This is a reminder not to forget about spin drift and know the direction of your rifle's twist.
What was your cartridge and load? What was your come up for 960 yards?
 
Last edited:

Canhunter35

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I’ve done that on a stubborn coyote. Now I dial spin drift when I dial elevation and hold for wind unless I’m having to hold more wind than my reticle has hashmarks
 

Litehiker

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I was shooting a Ruger Precision Rifle.
1:8 twist
24" barrel
Velocity- approx. 2,700 fps.
Hornady 147 gr. ELD-M (factory load)

I think 1:7.5 or 1:7 would be better for stabilizing 147 and higher weights and my next barrel, a stainless %R Bartlein will have a 1:7.5 twist.
Usually I shoot 140 gr. Hornady bullets W/40.6 gr. of H4350, CCI LR primers.

Eric B.
 

Dog Rocket

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I was shooting a Ruger Precision Rifle.
1:8 twist
24" barrel
Velocity- approx. 2,700 fps.
Hornady 147 gr. ELD-M (factory load)

I think 1:7.5 or 1:7 would be better for stabilizing 147 and higher weights and my next barrel, a stainless %R Bartlein will have a 1:7.5 twist.
Usually I shoot 140 gr. Hornady bullets W/40.6 gr. of H4350, CCI LR primers.

Eric B.
Your spin drift at that range should have been right at .154 MILS, which is 5.3".

Your wind call was spot on. **** happens when you shoot small stuff really far away.
 

Litehiker

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Now I'm going to re-do the small dope sheet I have taped on my bubble level bracket so I have spin drift values (for the 140gr. ELD-M bullets) in a color coded column beside each yardage mil come-up number.

Why the hell did I think I could always remember the values??
Eric B.
 

Dog Rocket

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Now I'm going to re-do the small dope sheet I have taped on my bubble level bracket so I have spin drift values (for the 140gr. ELD-M bullets) in a color coded column beside each yardage mil come-up number.

Why the hell did I think I could always remember the values??
Eric B.
Here is a little trick. Spin drift is between 1% and 2% of your drop, depending on the specific cartridge and twist rate.

I figured yours at 2% and was pretty d**n close to what you reported. The remainder could have easily been just the round falling to the bad side of your natural dispersion at that range. Most of the ballistic calculators add too much.
 

Litehiker

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Dog Rocket, good to know that 1% to 2% of drop.
According to Hornady's 4DOF ballistics the "angle of repose" (bullet angled a bit up from trajectory path) is what causes the air to catch the front end of the bullet and make spin drift. I can see how that would change with drop angle.

Eric B.
Gotta quit thinking this too much. I think my brain is full. ;o)
 

Dog Rocket

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Dog Rocket, good to know that 1% to 2% of drop.
According to Hornady's 4DOF ballistics the "angle of repose" (bullet angled a bit up from trajectory path) is what causes the air to catch the front end of the bullet and make spin drift. I can see how that would change with drop angle.

Eric B.
Gotta quit thinking this too much. I think my brain is full. ;o)
It has been a while since I really dug into the specifics, I'm not all that good with this stuff anyway. I'm smart enough to learn what I need to learn, but lazy enough that I boil it down to what I can take away and forget the rest fairly quickly.

It was a conversation with Jim Boatwright about his latest papers that provided the nugget that spin drift, for any given bullet, was a predictable percentage of drop. That percentage, while potentially different for every bullet / rifle combo, fell most commonly between the 1% to 2% range.

Jim's paper was exploring the effects of hyper-stabilization of solid projectiles in the 20 turns per caliber range. However, in doing so it used data from extensive gov't 6DOF testing of bullets in the 28 to 32 twist per caliber range. This is the range of twist where most cup and core lead bullets are spun.
 

jrock

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Like the post and reminder that small things add up. I was at the range the other day and this guy with $10k of hardware on his bench came over to me and told me I needed to dial 0.25 MOA for spin drift at the 600 yard target I was shooting. Not sure if he had checked out the 2" group I was printing just to the right of center. Still a hit even thought I hadn't dialed for spin drift.
 

Wedgy

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Like the post and reminder that small things add up. I was at the range the other day and this guy with $10k of hardware on his bench came over to me and told me I needed to dial 0.25 MOA for spin drift at the 600 yard target I was shooting. Not sure if he had checked out the 2" group I was printing just to the right of center. Still a hit even thought I hadn't dialed for spin drift.
Tell him you have a left twist barrel.
 

Litehiker

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Dog Rocket,
With the great CD and BCs of the 140 gr. and 147 gr. Hornady ELD-M bullets I think the spin drift would be closer to 1% of the drop, which itself is not exactly a "rainbow".

Eric B.
 

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