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SPIN DRIFT!....I'm such a dummy

 
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Old 01-22-2011, 10:10 AM
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SPIN DRIFT!....I'm such a dummy

Well, well, well..... old dog learns new trick. I've never really paid much attention to spin drift. Over the years I have seen a few threads come up that asks about it and it seems the normal reply is that it "don't matter much"..... "too small a deviation to worry about", so I have always just kept the spin drift calculation turned off on my ballistics programs and ignored it. Why complicate things further if it is too little to matter???

I picked up a copy of APPLIED BALLISTICS FOR LONG RANGE SHOOTING (Bryan Litz) a few days ago. First, I was just browsing the book before settling in for a real in depth study and happened across the chapter on "gyroscopic drift". Bryan makes a few statements about how it is "subtle" effect, "extremely difficult to measure", so with my false security so confirmed, I moved on in my scan. At the end of the chapter I see a chart of the drift of a 750 grain .50 cal Amax out of a BMG. I have a 510 AM and that is the bullet I shoot so looked at the chart and it showed at 2500 yards the spin drift is over 81". NO WAY!, I thought..... at 1000 yards the chart shows almost 7". Now I am curious. I go fire up 4 different ballistic programs I have that do spin drift (including the one that comes with Bryan's book) and find much to my surprise that my favorite loads for my .338s all have 6" to almost 8" of spin drift at 1000 yards!!!

I have noticed for years that as I move out past 500 yards my groups tend to drift to the right.... the longer the range the further right. I typically have a little "left to right" wind on my range so I blamed that, and I got really obsessed about making sure my scope was level. Still, to ring my 1000 yard gong I kept having to dial .5 to .75 moa left windage correction in.

Now I know, spin drift is NOT something to be ignored. Not at ranges further than 500 yards anyway. I feel like such a DUMMY for letting this get by me for so long!!
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Old 01-22-2011, 11:19 AM
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Join Date: Dec 2010
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Re: SPIN DRIFT!....I'm such a dummy

No dummy RDM ...... you're probably 1 in 10000 shooters that has ever researched gyroscopic drift and actually know what it is! Fortunately, I trained for many years with the old timers from the long range 'Nam era who incredibly figured all the variables of long range shooting in their heads without the benefit of the iPhone ballistic programs that I use today. Ask someone today to read a mirage and watch them get a blank look.

Actually, it's kind of fun to play around with "spin drift". If you have the range that will accommodate it shooting a 3 shot group every 100 yds from 4 on out really emphasizes the whole drift phenomenon. I've found that if you limit the test to a zero wind day that each rifle and the same rifle with different bullet weights make a significant difference. For my "money gun" on friendly hunting shots or ringing the bell out to 1k the actual drift is 1.5" every hundred past 400. Most of the time I figure my wind, add or subtract the drift depending upon wind direction and let 'er rip.
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Old 01-22-2011, 11:41 AM
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Re: SPIN DRIFT!....I'm such a dummy

I have posted on this many times in the past here on this site. You can do a search about it---not rocket science but just simple physics.

horizonal stringing
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Last edited by Boss Hoss; 01-22-2011 at 11:49 AM.
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Old 01-22-2011, 12:34 PM
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Re: SPIN DRIFT!....I'm such a dummy

Quote:
Originally Posted by RDM416 View Post
Well, well, well..... old dog learns new trick. I've never really paid much attention to spin drift. Over the years I have seen a few threads come up that asks about it and it seems the normal reply is that it "don't matter much"..... "too small a deviation to worry about", so I have always just kept the spin drift calculation turned off on my ballistics programs and ignored it. Why complicate things further if it is too little to matter???

I picked up a copy of APPLIED BALLISTICS FOR LONG RANGE SHOOTING (Bryan Litz) a few days ago. First, I was just browsing the book before settling in for a real in depth study and happened across the chapter on "gyroscopic drift". Bryan makes a few statements about how it is "subtle" effect, "extremely difficult to measure", so with my false security so confirmed, I moved on in my scan. At the end of the chapter I see a chart of the drift of a 750 grain .50 cal Amax out of a BMG. I have a 510 AM and that is the bullet I shoot so looked at the chart and it showed at 2500 yards the spin drift is over 81". NO WAY!, I thought..... at 1000 yards the chart shows almost 7". Now I am curious. I go fire up 4 different ballistic programs I have that do spin drift (including the one that comes with Bryan's book) and find much to my surprise that my favorite loads for my .338s all have 6" to almost 8" of spin drift at 1000 yards!!!

I have noticed for years that as I move out past 500 yards my groups tend to drift to the right.... the longer the range the further right. I typically have a little "left to right" wind on my range so I blamed that, and I got really obsessed about making sure my scope was level. Still, to ring my 1000 yard gong I kept having to dial .5 to .75 moa left windage correction in.

Now I know, spin drift is NOT something to be ignored. Not at ranges further than 500 yards anyway. I feel like such a DUMMY for letting this get by me for so long!!
a good rule of the thumb is that for every .0001" in error with the C/G of a bullet; you'll see 3/32" at 100 yards
gary
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Old 01-22-2011, 01:22 PM
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Join Date: Jan 2008
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Re: SPIN DRIFT!....I'm such a dummy

Well its good to see folks are beginning to wake up and smell the coffee. I got run off over at snipershide a couple of yrs. ago by the illustriuos( Frank ) for suggesting such a thing existed. I think even he now admits it exists but maintains it just isnt worth worrying about.
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Old 01-22-2011, 01:58 PM
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Location: The rifle range, or archery range or behind the computer in Alaska
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Re: SPIN DRIFT!....I'm such a dummy

You are no dummy. We all 'let things slide by us' from time to time. For whatever reason we ignore it because we have never had to deal with it or there was some other variable cancelling it out. Sometimes we are just so overwhelmed with all of the varibles to factor in and we are frustrated with finding a good load etc etc..... that we are to fried to take on anymore 'problems'. It's called human nature. Welcome to the human race!!

I am glad you realize now the value of spin drift. It still suprises me that so many of the members here think its all a myth. That it is the product of a canted scope or unknown winds etc....They are only fooling themselves and setting themselves up for failure.

Thanks for sharing.

M
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Old 01-22-2011, 02:03 PM
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Re: SPIN DRIFT!....I'm such a dummy

Augustus, I wouldn't worry to much over the idiots at snipershyde blasting you. I went over there one day to look around and I can see where a guy without very much experience could get caught up in the BS over there. But a guy with quite a lot of experience can see immediately many of the guys that try to run that forum seem to be young kids and have a lot to learn.
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