## Spin Drift

Started by 375CT, March 03, 2014, 07:41:28 PM

#### 375CT

I know most programs rely on Litz's formula to compute SD.

My question is, does anybody honestly tested and/or verified its accuracy?

Problem is I don't have access to 1000 meters/yards range, so it's next to impossible for me to even attempt any serious test.

Any light on this is greatly appreciated.

#### mman

Litz's formula is good approximation for some projectiles while some, not that good. There is more about verification against 6-dof in Litz's book. Validation however is next to impossible since spin drift is so small compared to wind, cant and some other dispersion factors. Even if you could do the validation it would be valid for that projectile, velocity range and twist only.

All that being said litz's formula is pretty much good enough for all practical purposes. Whole spin drift is so small that if you can correct it even with accuracy of 30% you can pretty much forget the rest.

I have shot quite a lot for long range and only once I remember that conditions were so calm that validation could have been possible. I put 3 shots to 800 meter target with my 223 rem and 75 gr.  berger VLDs MV 880 m/s. I didn't do any horizontal correction. Group was about 0,5 moa and center of it just about were litz's spin formula had predicted. I had anti can't device installed and rifle was zeroed carefully but I don't know...Might still be just a coincidence .

mman has a 1000m+ range in his backyard! Shall we  ask him?

The problem, of course, is to do the measurements.

In those measurements one has to vary some quantities that affect spin drift and other factors that affect horizontal displacement. Then there is wind and measurement errors. To single out to spin-drift effects one must trust the proper computation of all other effects that deflects the bullet. I would guess that only a professional organisation is able to do the measurements.

As far as I know, the US Army is the only institution that published measurements.

Brian did the best next thing. He used a proffesional 6DOF ballistic program - PRODAS - and fitted the spin drift of bullets.

#### 375CT

Mman & Richard,

Thanks for the feedback. Well put, next to impossible, unless done in a wind tunnet I guess.

The problem with PRODAS is, it's another approximation, yet sophisticated but not quite the "real" thing as far as I've been told so far.

Like you guys said, it's very easy to get SD lost in the background noise.

For what it's worth, at least for the record, Litz's formula is surprisingly the same as the one published early bu a guy on the LRH forum...and nobody gave him the credit!