Spin Drift ???

[cncmill]

Guys, I am new to bench rest shooting and would like some help. I recently started shooting 1000 yards at our hunting camp since we put in a 475 and 1024 yard target . I had a 7", 10 shot group at the 1000 yard target. Dead on up and down but it was 8" to the right. I had 1 to 2 mph wind from about 11 0'clock. Is the 8" to the right more wind or spin drift? Is there a way to calculate spin drift? Thanks for any help you can provide.

6.5mm x 284
8.5 twist
140gr. Berger VDL
3030 fps.

 

[cmillard]

usually, spin drift makes the bullet go up and to the right. and at 1000 yards it would be minimal. the wind could have done that.

 

[kentuckytroutbum]

It could have been wind as cmillard said. Or it could have been the coriolis effect. Have you run the numbers through a ballistic program such as JBM or Bryan Litz's to see what the effect might have been? What magnetic direction where you firing?

Bill

 

[lpreddick]

on spin drift and bullet going "up and to the right"...a right hand twist(clockwise) bullet goes to the right and down slightly more than by gravity and drag...if i read brian litz book correctly. wind is usually the culprit at such long range. [/quote] if i can see you, i can touch you. BANG!

 

[Romulus]

Tagged a Litz snippet at the bottom.

Like Kentuckytroutbum queried, I am kind of interested in exactly what bearing your range is set up and what area of the world you are shooting in. I would like this to be the one instance where Coriolis effect really is responsible. It would make my day, and I would want to stop by someday for a visit to shoot.

Litz book has more applied info on the subject, but I just dropped my book off for a friend to borrow for a bit : (

http://en.wikipedia.org/wiki/External_ballistics#Coriolis_drift

Can anybody run a worst case scenario on Coriolis at 1024 yds at the north pole, NYC, DC, and Houston? That would be handy to have just for BSing purposes.

http://www.appliedballisticsllc.com/index_files/SpinandCoriolisDrift.htm

 

[kentuckytroutbum]

Romulus-

Thanks for posting the link to Brian's chapter. Some time ago, I had watched a video on the coriolis effect on long range weapons fired in the primary directions. The "stationary" target and the earth were shown to be rotating eastward in relation to the trajectory of the projectile, and the showed the POI at the moment of impact. Very enlightening.

Bill

 

[cncmill]

Thanks guys for all the input. After looking at my Nightforce ballistic software a little more, I found it does have inputs for spin drift and the Coriolis effect. Spin drift, I will have to figure out in a "no wind" situation to plug into the software and rough figures for the Coriolis effect is about 3.2in right and .5in up @1000. The software asks for Latitude (deg.) and shooting direction (deg. from North) to figure it out. I am guessing at the direction of the range right now. The next time I am there I will look to see exactly what direction it lies in and get the exact numbers. It looks like the wind + Spin drift + the Coriolis effect + me is giving me the 8" to the right. I guess that is what makes it so much fun. The range is in Sullivan County, PA near Eaglesmere.

 

[jerrschmitt]

Rather than blame Coriolis effect or spin drift, I would look at scope cross hair alignment and tracking of the scope first.

Not saying the coriolis and spin aren't real world problems, but that's a lot of miss for those factors and it's very easy to be off that far with just a small misalignment in cross hair plumb.

 

[Nomad47]

For my 243 wssm shooting 105 AMax bullets at 3250 MV, spin drift is equal to a .9 MPH wind. In other words, entering a .9 MPH right to left wind in the JBM spin drift calcs yields a near perfect zero drift for all ranges.
HTH.
Bill, the Retired Nomad