Spin Drift Calculation?

[Terry]

Is there a formula for calculating spin drift.

For example: 6mm 107 sierra, 3,000 fps, 1:8 Rt hand twist.

Is it possible to calculate the spin drift at 600 yards and beyond?

 

[butchlambert]

Send Bryan Litz a PM or email, or better yet, buy his book.

 

[js223]

I will second the suggestion to buy Bryan's book. Applied Ballistics For Long Range Shooting came with software that has the ability to calculate spin drift. You will need to measure the length of the bullet that you wish to calculate for.

 

[Bryan Litz Ballistics]

The formula for spin drift is pretty easy, if you know your stability factor (SG).

SG is more difficult to calculate, but fortunately there are online calculators that can do this, such as the one on the Berger site: http://www.bergerbullets.com/twist-rate-calculator/

So, after you get your bullets SG, plug it into this formula for spin drift:

For example, if your SG is 1.8, and the tof to 1000 yards is 1.6 seconds, the spin drift would be:

SD = 1.25*(1.8 + 1.2)*1.6^1.83

SD = 8.9 inches.

-Bryan

BTW, the screenshot above is from Chapter 6 of: "Applied Ballistics for Long Range Shooting", if you're interested in more detail.

 

[Forum Boss]

Wow -- Thanks for the fast, definitive answer Bryan.

I think our Forum members may soon get spoiled, having their ballistics questions answered by a bonafied rocket scientist. Imagine if one could got to an automotive Forum and have questions answered by McLaren's top Formula 1 engineer.

We are lucky....

 

[butchlambert]

Wow -- Thanks for the fast, definitive answer Bryan.

I think our Forum members may soon get spoiled, having their ballistics questions answered by a bonafied rocket scientist. Imagine if one could got to an automotive Forum and have questions answered by McLaren's top Formula 1 engineer.

We are lucky....

Paul,
I don't think the McLaren guy could relate to our automobile problems, but I understand the point that you are trying to make.

 

[Terry]

The formula for spin drift is pretty easy, if you know your stability factor (SG).

SG is more difficult to calculate, but fortunately there are online calculators that can do this, such as the one on the Berger site: http://www.bergerbullets.com/twist-rate-calculator/

So, after you get your bullets SG, plug it into this formula for spin drift:

For example, if your SG is 1.8, and the tof to 1000 yards is 1.6 seconds, the spin drift would be:

SD = 1.25*(1.8 + 1.2)*1.6^1.83

SD = 8.9 inches.

-Bryan

BTW, the screenshot above is from Chapter 6 of: "Applied Ballistics for Long Range Shooting", if you're interested in more detail.

Thanks!

Terry

 

[cdngary]

Is spin drift always a predictable right movement of the bullet {in Right twist barrel} at the different distances that can be added or subtracted from the wind drift at the different yardages that a person might be shooting at. From the eg. would the 8.9 in. be added to a left windage and subtracted from the right.

 

[Bryan Litz Ballistics]

Yes. Spin drift is independent from wind, and adds or subtracts to windage.