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spin drift & Coriolis effect

 
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  #29  
Old 03-22-2011, 04:46 AM
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Re: spin drift & Coriolis effect

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Originally Posted by load View Post
actually it does alway cause an impact right unless you are in the southern hemisphere in which it will ALLWAYS be LEFT. just as a left spin will cause left drift.
if you dont beleave me shoot out to 1760 yrds and do as you describe. you WILL be WAY off if you try to adjust for a impact left corialis in the northern hemisphere no matter the direction u shoot.

the adjustments i list will get u closer than u can dial anywere in the usa(except alaska), given mean velocity of 2800 fps (a good median for most types of rifles)and bc in the high 5's
At least a couple times a year we shoot our .50's out to a little over 1600 meters, the same mile as your 1760 yards. As a matter of fact that's what we have scheduled this weekend.
When talking about the effects on REALLY long range shooting I believe that I have some pretty knowledgeable resources on the subject. I not only asked our Mortar Platoon SGT (a 20 year 11C), but also five 11C instructors(all MLOC grads, with close to 100 years combined experience).They all said the same thing, you cannot accurately account for Coriolis without a Mortar Ballistic Computer and input the gun's coordinates, the target's coordinates and factor in the bearing from gun to target. You cannot just say it will always land to the right. And these guys shoot out to 7200 meters. I also have a few buddies that are instructors at the Sniper School and they will say the same thing I was told going through the school, you cannot just range a target and put a number on it...you can't just favor/adjust right and call it good.
Now then, the one common factor these guys have in common is they are all Army trained. That could be the down fall of my/their experience...but I doubt it. It if was that easy I bet we would be using it.
As for the 3" right spin drift and 3" right of Coriolis at 1000 yards. I would be willing to say that it's really 6" of spin drift. Looking at the data book for our .50's shooting the MK211 rounds, it shows a .5 MOA of spin drift @ 1000 (also what I use for my .338 Edge). For our M24's most guys I know use .75-1 MOA.
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  #30  
Old 03-22-2011, 08:25 AM
rem rem is offline
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Re: spin drift & Coriolis effect

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Originally Posted by Imho View Post
In Excel the formula for the cell would be:

=1+2*0.00007292*2870/-32.2*COS(RADIANS(45))*SIN(RADIANS(270))

thanks, that is very helpful.
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  #31  
Old 03-22-2011, 08:42 AM
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Re: spin drift & Coriolis effect

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Originally Posted by Bravo 4 View Post
......As for the 3" right spin drift and 3" right of Coriolis at 1000 yards. I would be willing to say that it's really 6" of spin drift. Looking at the data book for our .50's shooting the MK211 rounds, it shows a .5 MOA of spin drift @ 1000 (also what I use for my .338 Edge). For our M24's most guys I know use .75-1 MOA.
Bravo 4,

You are exactly right about just the spin drift being way more than 3" at 1000 yards. According to Bryan Litz book, Applied Ballistics for Long Range Shooting, using two different equations for a .308, 168 grain SMK the spin drift alone will be somewhere between 11.1" to 14.24" with a 1:10 twist barrel, a little less with a 1:14 twist. This would be somewhere between 1 to 1 1/2 moa. The spin drift for a 50 BMG using a 750 grain Amax, 1:15 twist barrel is 6.73" as measured by the military using Doppler Radar. (very accurate data). I have verified this with my 510 Allen Mag, using the same bullet but at a higher velocity (the test was a 2690 and I am running 2850 fps) I am seeing 6" right drift at 1000 yards ( a little over 1/2 moa). I use 1/2 moa for my 338 Khan at 1000, the ballistics program I am using gives it 6" value.
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  #32  
Old 03-27-2011, 04:57 PM
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Re: spin drift & Coriolis effect

I've read this entire thread and am more confused than before I started. First , Corioles has nothing to do with spin drift. It is all about the rotation of the earth and is a very small factor in 1000 yard shooting It is dependant on time of flight and direction. North and south have no effect and east will be high poi and west will have low poi. How much depends on distance and time of flight.
Spin drift is caused by the effect of gravity on the moving ,spinning bullet . when you apply a force to a gyroscope it tends to move at right angle to the force applied. On a bullet spinning right and gravity pulling down, the drift is to the right. The opposite for a left spin.
When you have a wind blowing from the left , the gyroscopic force will make the bullet go right and down. Conversely a wind from the right will make the bullet go left and up. I know many will argue this but real world experience tells me that i should listen to my college physics lessons and the results I have shooting long range cold bore shots. The drift is about 3 1/2 minutes to 1 . You don't get spotter shots on game. Try it next time you shoot in the wind.

Next time you shoot a spotter shot in the wind, , pay attention where it hits. Wind from the left, hit will be low. Wind from the right, the hit will be high!

Found a really good old thread with some actual testing. Vertical Component of Wind Drift

Last edited by paphil; 03-27-2011 at 08:01 PM. Reason: more thoughts added
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  #33  
Old 03-28-2011, 08:25 PM
rem rem is offline
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Re: spin drift & Coriolis effect

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Originally Posted by bsl135 View Post
Here's another resource for spin drift and coriolis effect:
spindrift
-Bryan
Bryan did you come up with all of those equations? I like your article.
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  #34  
Old 03-29-2011, 12:28 PM
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Re: spin drift & Coriolis effect

Good points bravo.

modern ballistic programs for shooters are nothing more than old mortar & artillery math programs tweaked specifically for the rifle shooter.

Its from this that I got my 3" & 3" reference.
Per your spindrift quotes: spin drift varies with diameter of the projectile, length of projectile and RPM, but the program will not ask for rpm because you will not know... it asks you for muzzle velocity and rifling twist and calculates it for you.

All good stuff!
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  #35  
Old 03-29-2011, 01:31 PM
rem rem is offline
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Re: spin drift & Coriolis effect

Quote:
Originally Posted by paphil View Post
I've read this entire thread and am more confused than before I started. First , Corioles has nothing to do with spin drift. It is all about the rotation of the earth and is a very small factor in 1000 yard shooting It is dependant on time of flight and direction. North and south have no effect and east will be high poi and west will have low poi. How much depends on distance and time of flight.
Spin drift is caused by the effect of gravity on the moving ,spinning bullet . when you apply a force to a gyroscope it tends to move at right angle to the force applied. On a bullet spinning right and gravity pulling down, the drift is to the right. The opposite for a left spin.
When you have a wind blowing from the left , the gyroscopic force will make the bullet go right and down. Conversely a wind from the right will make the bullet go left and up. I know many will argue this but real world experience tells me that i should listen to my college physics lessons and the results I have shooting long range cold bore shots. The drift is about 3 1/2 minutes to 1 . You don't get spotter shots on game. Try it next time you shoot in the wind.

Next time you shoot a spotter shot in the wind, , pay attention where it hits. Wind from the left, hit will be low. Wind from the right, the hit will be high!

Found a really good old thread with some actual testing. Vertical Component of Wind Drift

i have read the same thing some where else( wind with spin drift) and it makes complete sense. but does it effect it a lot or a little?
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