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spin drift

 
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  #8  
Old 02-17-2008, 04:16 PM
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I did not say that it was difficult to see the effects of spin drift, that is around 1 moa at extended ranges in my experiences, I said it was difficult to see the effects of the earths rotation due to it being so small. David Tubbs has went into great detail concerning both of these subjects on another site. He does believe that spin drift exists and should be compensated for. The bottom line is if you are shooting alot and you are not drifting right at extended ranges in calm conditions then dont worry about it, on the flip side If I dont correct for it I will hit about 1 moa right at 1000 and beyond. As stated when this subject came up, it is usually a volitile subject and you either believe it or you dont.
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  #9  
Old 02-17-2008, 04:49 PM
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Rustystud, the Coriolis has nothing to do with which side of the equator you are on, it does not matter if you are 30 deg. N or 30 deg S. the effects are the same,it does matter which direction you fire the round from that location. The effects will be different at different latitudes, anyway this is mostly gee-whiz info because a shooter wont need to worry much about it until you get way out yonder.
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  #10  
Old 02-18-2008, 11:03 AM
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I recently did a bunch of testing with diffferent calibers at distance to determine the effects of spindrift at the request of Nightforce scopes. I mostly confirmed what I already knew about spin drift.
1. Spin drift exists in all calibers, some so slight as to not be practically detectable, others quite a bit.
2. Spin drift will be effected differently in the same caliber by velocity, twist rate, bullet profile and atmo conditions.
3. Spin drift is predictable after some testing, after determining the spin drift (this is not real easy) at a given distance like 1000 yards other distances can be predicted by entering it as a wind function. After determining 1000 yard drift I was able to predict 1500 + drift using the wind drift function and my 1000 yards info as a validation number. It will be a small number usually less than 1 mph of wind.
4. The longer and higher BC a bullet the more suseptable it is to spin drift.
5. Proper twist rates reduce spin drift but don't eliminate it.
6. If spin drift is roughly 1 mph or less in most calibers few can estimate the wind that accurately. This doesnt mean that you should discount spin drift as it is, at distance one of the small compounding errors that can make a miss. I believe that these misses are more often interprited as wind estimate errors.
7. Every single change in platform can effect spin drift differently, velocity, twist etc.
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  #11  
Old 02-18-2008, 01:49 PM
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The algorithms that would be required to properly account for spin drift would be so complicated that accounting for the various types of rifling, air temperature, density etc that it would be a veritable science project but it could be done with enough money and time…

Yes it is a factor in the world that I compete in.
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  #12  
Old 02-18-2008, 04:05 PM
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Shawn, thanks for taking the time to share the info in on this, I just about went crazy when I first started shooting 1000+. I was shooting a 50 cal. with 750 A-maxs. The combo shot pretty well and would group decent but it would hit dead on at 100 yds, about 3 in right at 600 and the longer the distance the worse it got. I had never heard of spin drift at the time and I thought I must be canting the rifle or the reticle wasnt plumb or some other error I was committing. I bought a base level, got everything plumbed and had the same problem only a little worse. I then noticed the 300 ultra was doing about the same thing just not as bad. Yep, the 308 does it too. I discussed this with quite a few people and was told everything you can imagine. Most said it was caused by undetected wind, some said I just couldnt shoot, many went into great detail about how I wasnt getting a good cheek weld or I needed to curl my little toe on my left foot just prior to breaking the trigger. I began to think it was because I was born naked when I was a baby. Finally I came to the realization that something was causing the projectile to drift to the right. I would get up early in the morning and tie little white feathers from the posterior of a chicken all up and down the range, located where they could be observed while engaging the targets at various ranges. What I learned cost a young fortune, Shawn just got through telling everyone in less than a minute what took me a year, a barrel, and who knows how much money to find out. A pure target shooter will never figure this out, because he will go to the range, shoot a sighter, and correct for windage. The rest of the day will be spent compensating for changing conditions. This works great for their purposes but for someone needing to make first round hits on littlebitty things way out there, spin drift is definately part of the equation. I agree with the gentleman in the prior post, a program that could figure this out for every round loaded with every make, shape, or form bullet would take some doing; however any individual can sort this out without a lot of pain for the combination he is shooting. Do the numbers that Shawn posted and see if they are significant in your situation. If not, dont worry about it. If you are shooting at game 800+ you should know how much it affects you and compensate for it. By Shawns numbers it effects my 300 ultra about 7 1/2 inches at 1000. This is very close to what I have seen. That in itself is the difference in a clean kill and a good day gone bad.
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  #13  
Old 02-18-2008, 08:10 PM
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While it might be possible to build a program to determine spin drift for a given combo, I think it would be nearly impossible to run all the possible combos available. While testing the 300 WM 190 gr there was a noticable difference in a 10 twist barrel vs a 9 twist. Also there was a subtle but consistance difference between 2 1-10 twist barrels of different rifling profiles. I believe that spin drift is an important factor at distance but truly requires some testing and practice to deal with. Once again even if we had a program to determine SD it would still be just a place to start field testing. Once you have the effective wind speed figured out to equate the SD, it has proven to be an accurate method of prediction in the testing I did from 1800-6600ft, 35-80 degrees, 30-80% humidity in various combinations and distances out to just over 1500 yards. In the Nightforce ballistics program I simply entered .85 mph right wind into the base chart and saved it. If I had an estimated wind of 6mph left, I entered 5.15 mph left. This provided for some truly outstanding first round hits on rocks of the 10-12" range at 1500+ yards. Without the SD compensation the would have mostly been close misses written off to slight wind estimation errors. It can drive you nuts but I really dig this stuff.
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  #14  
Old 02-18-2008, 08:40 PM
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It never occurred to me to deal with this as a function of wind. This is gonna be GOOOOOOD.
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