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Spin Drift

 
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  #1  
Old 09-29-2013, 05:30 AM
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Join Date: Mar 2012
Location: Elk Garden,WVa
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Spin Drift

I've been reading on spin drift. And I have a question I hope someone can answer.

With spin drift being somewhere around 8-10 inches at 1000 yds. What is is going to be at 100,200,300 and so on?
My way of thinking was that the instant the bullet leaves the barrel it begins its rotation to the right. Which means if I corrected my point of impact at say 200 yds, it puts my flight path back in line with the bullseye. Which I know from experience doesn't hold true. But at what point does a person need to begin to allow for the spin drift?

Example- Do I not hold over until say 500 yds, then begin to compensate accordingly after that until I get to the 8-10 inch hold over at 1000?

P.S- I am also aware of the 2.5-3 inch difference in point of impact the earths rotation has at 1000. Just so we don't get side tracted on the spin drift question.
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Old 09-29-2013, 07:30 AM
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Join Date: Dec 2012
Location: Charleston, WV
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Re: Spin Drift

Quote:
Originally Posted by oldmossy View Post
I've been reading on spin drift. And I have a question I hope someone can answer.

With spin drift being somewhere around 8-10 inches at 1000 yds. What is is going to be at 100,200,300 and so on?
My way of thinking was that the instant the bullet leaves the barrel it begins its rotation to the right. Which means if I corrected my point of impact at say 200 yds, it puts my flight path back in line with the bullseye. Which I know from experience doesn't hold true. But at what point does a person need to begin to allow for the spin drift?

Example- Do I not hold over until say 500 yds, then begin to compensate accordingly after that until I get to the 8-10 inch hold over at 1000?

P.S- I am also aware of the 2.5-3 inch difference in point of impact the earths rotation has at 1000. Just so we don't get side tracted on the spin drift question.
With my 6.5-284 I zero it 1/2" to the left at 200 yards. That puts me dead on for spin drift at about 575 yards and I only have 3" of spin drift to the right at 1000 yards. This is a hunting gun with most shots being 800 or less. At 800 I only have 1-1/2" of drift which isn't much. So I pretty much only have to adjust for wind on game. Hope that helps.
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  #3  
Old 09-29-2013, 07:47 AM
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Join Date: Sep 2013
Posts: 3
Re: Spin Drift

Use a ballistics calculator. You need to know your muzzle velocity and the BC of the bullet you're shooting as well as the twist of your barrel.
My setup only drifts 2" at 5, but yours will be different due to BC, velocity, and barrel twist.
Once you see the data, you can decide when you need to hold off.
JBM - Calculations - Trajectory (Drift)
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  #4  
Old 09-29-2013, 07:47 AM
BMF BMF is offline
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Join Date: Jul 2008
Location: Near Fowler/St Johns...Michigan
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Re: Spin Drift

Go to the G7 ballistic calculator ( near the top of this page ) and check out the numbers. It's all there. Just input your data, with or without spin drift, to see the affect.
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  #5  
Old 09-29-2013, 08:57 AM
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Re: Spin Drift


just get a can Spin d
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  #6  
Old 09-29-2013, 10:22 AM
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Join Date: Mar 2012
Location: Elk Garden,WVa
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Re: Spin Drift

I'm going to try and present my question(theory) in the way I presented it to my longrange buddy at work. To which he replied-You're thinking about it wrong, it doesnt work that way.

In a perfect word. No spin drift, and no wind- Your muzzle and scope is lined up perfectly with bullseye at 1000 yards. You fire, and it hits perfect in bullseye.
Now- Add spin drift. You hit 8-10 inches to right.

Now-It's looks to me like spin drift at 100 yds should be 1 inch. 2" at 200 and so on.

So if I were to adjust my scope to perfect at 100 or 200 or 300 or whatever, I would be adjusted for spin drift out to whatever distance. In my mind, spin drift to the right begins the moment the bullet leaves the barrel. So when I sight in my rifle, I've already compensated for spin drift.

But I'm under the impression it doesn't work that way, because the spin of the bullet doesn't slow down as fast as the speed of the bullet(fps). Which translates to the bullet having more time to drift right because it isn't covering as much ground as it does closer to muzzle.

That's where my original question came from.

Looks to me if I sighted in 1/2 inch left at 200(which is what my buddy told me to do also) I would be overcorrecting for the spin drift, and would actually hit 5" left at 1000.

I personally see my impacts are right of my point of aim when I shoot 1000, and I am dead on at 200. I'm just trying to get a feel for at what point this spin drift needs to be compensated for.

I'll check out that ballistics chart you guys mentioned.
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  #7  
Old 09-29-2013, 10:37 AM
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Re: Spin Drift

Shootnmathews,
What is your point of impact at say 400 yds. I'm assuming you are 1" left?
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