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

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by jmason, Apr 23, 2009.

  1. jmason

    jmason Well-Known Member

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    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.
     
  2. phorwath

    phorwath Well-Known Member

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    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,
     
    Last edited: Apr 23, 2009

  3. jmason

    jmason Well-Known Member

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

    BryanLitz <b>Official LRH Sponsor</b>

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    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
     
  5. Coues Sniper

    Coues Sniper Well-Known Member

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    Glad to see you're still around Bryan :cool:
     
  6. Michael Eichele

    Michael Eichele Well-Known Member

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    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.
     
  7. jmason

    jmason Well-Known Member

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    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.
     
  8. AJ Peacock

    AJ Peacock Well-Known Member

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    Long answer short.

    CE has both vertical and horizontal components. The vertical component depends on the direction you are shooting, the horizontal does not. If you have a right hand twist barrel, the CE and SD add up to move the bullet to the right, there is really no reason to distinguish how much of each component is at work (as long as you compensate for their affect on your shot). If you have a left twist barrel, CE will counteract a little of the SD (around 1/3 of it).

    Read Bryan's explanation, it is very good.

    AJ
     
  9. noel carlson

    noel carlson Well-Known Member

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    JMason,

    AJ is correct, in that as long as you do not change test latitude to any significant degree, there is no practical need to distinguish the two effects. They are essentially constants relative to horizontal shift.

    If you wanted to separate the components empirically (for the fun of it:)), reverse barrel twist, and get a statistically meaningful sample of gauging hits from the two barrrels at range.

    The median offset, from an optically established center, equals the coriolis deflection.
     
  10. phorwath

    phorwath Well-Known Member

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    I look forward to your post on the SD measured at 1000.

    I don't think you can do any better than to rely on Exbal for the Coriolis Drift. Coriolis drift is like gravity. It's a scientifically known commodity and if you plug in your correct latitude (like 45.4 degrees North or whatever) into your software, you should obtain a very accurate coriolis drift number. So using the ballistic software coriolis drift is the way to go.

    The art of mathematically determining spin drift is less precise. So spin drift is what you'll want to determine by field measurements like you're doing now.

    Plus as has been mentioned, there are other sources of extraneous drift that our outfits and we as shooters can cause unless we've got everything perfect. So when you go to measure spin drift, you may be measuring a little rifle cant or other imperfections also. But at least you'll then know what kind of drift you're getting from your rifle setup. If your measured spin drift seems reasonable then you're good to go and can allow for it on long range shots going forward. If the spin drift measured is out of whack like mine was with one of my rifle setups, then you can trouble-shoot your equipment and get it corrected. In my case, the scope was mounted pointing off to the right from the direction of the barrel and I had to remount the scope bases to get that corrected.

    Now I take a long straight edge when I mount scope bases to the receiver. I place the straight edge along the edge of the scope base on both sides of the scope base(s) and see where it lines up over the end of the muzzle. I do that while refining the mounting position of the scope base until I get equal positioning over the end of the muzzle - at that point I know my scope base is mounted parallel to the barreled action. Wish I had that trick in my bag all along.

    Good luck and please post again and let us know what you determine for spin drift at 1000 yds. Pick a still day to shoot and you'll get it nailed down.

    PS: The coriolis drift does have a vertical drift component if you're shooting east or west. If you shoot due north or south - no vertical component. However you're only concerned with the horizontal component of coriolis when distinguishing between coriolis drift and spin drift, because spin drift is only horizontal, no matter which azimuth (direction) your shooting.
     
    Last edited: Apr 24, 2009
  11. jmason

    jmason Well-Known Member

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    AJ- I know you use Exbal (don't know which release)so you should be able to make sense of this. I have the most recent release (there is a chance I missed something in the software) and to determine SD it must be measured and input into the program for it to calculate at various ranges. Once I measure these at distance and input the data then I can forget about the horizantal portion of CE funtcion in Exbal? From the data I gave it on my SD will it now incorporate both spins by using that data?

    The CE function comes up as a separate page and is not automatically incorporated in the drop data like SD can be. So lets say I measured my SD and input it in Exbal, then I decide to shot at 1200yds. I put in my conditons, range, latitude, and target direction. The out put will not include CE. I have to go to "options" and select CE then deduct the range info on that chart from my target engament chart if I want to use the verticle portion of the CE.

    phorwath- I'll gladly post what I find, it may be a week or two though.
     
  12. AJ Peacock

    AJ Peacock Well-Known Member

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    I don't use those functions in Exbal. My favorite method is to ignore SD/CE. By doing my zero wind adjustment at 800yds or further, I reduce the SD/CE error at longer ranges and ignore it at the closer ranges.

    For my shooting, I like an 800yd zero wind setting. This setting has a small effect at 600yds (only about 1/4moa left ) and (1/2moa left at 200yds), so it doesn't really come into play for hunting.

    By using my zero at 800, my drift at 1000 isn't really noticeable. At 1400yds with my 338AM, I get around 1/2moa drift from my 800yd zero (I wish I could judge wind within 1/2moa at 1400yds!). I just take that into account in my head when calculating my wind drift.

    Hope that helps.

    AJ
     
  13. jmason

    jmason Well-Known Member

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    That helps me understand your point of view much better. I guess I like to do things the hard way. I don't want a long range wind zero. I want to dial everything. The last time I shot paper at 600 I had about 1/3 MOA right side impacts from my aim point. That aligns to some degree with what you see at 600 and you long range wind zero.

    Am I making sense to anyone with what I'm trying to do/explain. If I am Paul's 1st reply in this thread would make a short story of it correct?

    It's hard sometimes to think my thoughts clearly let alone type them for others to understand. :D
     
  14. jmason

    jmason Well-Known Member

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    I posed some questions to Gerald Perry and I am posting what he replied. BTW seems like a pretty good guy. The black is me and red was his reply.

    I have the most recent release of EXBAL for PPC (there is a chance I missed something in the software) and to determine Spin Drift it must be measured and input into the program for it to calculate at various ranges. Once I measure these at distance and input the data, then I can forget about the horizontal portion of Coriolis Effect function in Exbal? From the data I gave it on my SD will it now incorporate both spins by using that data? Not exactly. Coriolis is not a spin it is a deflection. Spin drift occurs no mater what the azimuth of your line of sight. So that only the Coriolis deflection for the azimuth of your measurement of SD will be accounted for.

    The CE function comes up as a separate page and is not automatically incorporated in the drop data like SD can be, correct?
    That’s correct. It should not be a problem unless you are shooting artillery .

    Lets say I measured my SD and input it in Exbal, then I decide to shot at 1200yds. I put in my conditions, range, latitude, and target direction. The out put will not include CE. I have to go to "options" and select CE then deduct the range info on that chart from my target engagement chart if I want to use the vertical portion of the CE, right?Yes although generally the Coriolis displacements are within the accuracy limits of the finest long range rifles.