A new spin drift and coriolis question (I think) Exbal

jmason · #1 04-23-2009, 12:22 PM

I know this topic has been discussed at length but I'm not sure this was covered.

How do you determine what is Spin drift and what is Coriolis?
Here is an example to set a basis for the question.

When setting up your ballistic software (mine is Exbal) you need to measure your spin drift at 5 or 6 hundred yards in calm conditions. OK no problem. If you use the Coriolis effect option in the software with a latitude of 45 and a target direction of 270 @ 600 yards I see that my bullet will impact .9" to the right and .9" low.

Do I assume that Exbal's coriolis calculation is correct and subtract what it gives me from the spin data I collect? If I add spin on top of that I know it can't be right.

phorwath · #2 04-23-2009, 12:33 PM

I'd say yes. What you obtain on target is spin drift plus coriolis drift, and they're both rightward drift with right-hand twist rifling in the northern hemisphere. If Exbal gives you the coriolis drift, subtracting that coriolis drift from your total drift leaves you with spin drift.

That's the short answer. Other factors can cause drift also such as a scope not mounted parallel to the bore or a canted bore at the time of fire. But presuming no other factors have created other sources of drift, I believe the above "short answer" to be correct. Cheers,

jmason · #3 04-23-2009, 01:22 PM

Thank you my friend! I know you just went through deciphering this not too long ago. That makes you an excellent resource!

bsl135 · #4 04-23-2009, 03:50 PM

J,
Here's a link to my write-up on spin drift and coriolis effect. At the bottom of the article are links to examples showing how to calculate coriolis (horizontal and vertical). You can perform this calculation and know the contribution of coriolis.
The answer given above sounds right, this is just a way you can double check it if you want.
Take care,
-Bryan

Coues Sniper · #5 04-23-2009, 06:12 PM

Glad to see you're still around Bryan

Michael Eichele · #6 04-23-2009, 09:54 PM

In my opinion, spin drift AND coriolus change a wee bit in differing conditions. However, not to the same extent that drop is. So what I am saying is that SD and CE are a farily concistent issue.

Forget the calculator, get out there and test it to see how much there is and use that info rather than the calculator.

The calculator will be more accurate than you, your perception of the conditions, and (this is a big one) how plumb your scope was mounted.

Once you determine how much combined right hand impact you see, you can compensate for these issues more reliably than your software can tell you. This is because your software has NO idea how plumb your scope is. There is "I think it is plumb" and there is "It really is plumb".

Let the paper tell you and not the software. Leave the software for calculating drops values and wind values. Then you can add or subtract these other factors.

Of course, this works out to 1K. Once you step out to 2K, these variables become MUCH larger, to the point where software utilizing CE and SD is usefull.

jmason · #7 04-24-2009, 08:22 AM

Thanks again guys for the input. I know my scope is plumb. I have drawn a plumb line on paper @100 and dialed enough elevation to get me past 1000. The bullet impacts stay on the line. I haven't read Bryan's link yet (it may answer this). How is it possible to differentiate between SD and CE without using something as a given? In my case I used the software calcs as my given for CE and will measure SD. I have measured my spin once @ 600 but will do agian here and farther to validate it.