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spin drift

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Old 02-18-2008, 09:21 PM
Platinum Member
Join Date: Jun 2001
Location: Potomac River
Posts: 5,057
I went back and recalculated a shot from last season with the 240 Wby that I couldn't understand how it could have been so far over to the right even given the wind error which I measured after the shot. The 30 degree shift in wind direction plus 0.5 MOA spin drift (1.0 mph of wind) puts the bullet right where it landed.

Simply amazing what you can learn on this forum.
The Smokin Fur Rifle Club
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Old 02-18-2008, 09:40 PM
Silver Member
Join Date: Mar 2006
Location: SC
Posts: 222
Shawn, Augustus,
Man, Thank ya'll for this great info. I have not been shooting at long ranges for very long but here in South Carolina we do get some days will little to no wind and I knew something funny was going on. I'm shooting a 6.5 RUM with a 120g BT at 3830fps. muzzel velocity. The barrel is a Hart with a 1-12 twist. Saturday the wind was pretty calm, no noticible wind at all. I have a Zeiss with the .5cm @ 100m and I had 5 clicks left dialed in. I hit the taget in the middle with one being dead center the target at 1000 yards! I stated shooting at 500 and then 600 before going on to the 1000 and noticed that i had to add more and more left to the scope. Thanks alot for the help.
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Old 02-18-2008, 10:10 PM
Join Date: Jun 2007
Location: North Idaho
Posts: 1,822
It occured to me to deal with the SD as a wind function while doing the testing for NF. I was pouring over the SD results at known ranges and looking at predictability. It occured to me that plugging in the correct wind speed will closely mirror SD and change appropriately with atmosphericial changes. From my data and testing it is not perfect but within an 1" or so at most distances out to 1500+. I based my speed number on hard data from 1000 yards and validated it out to 1500 and some change. I'd like to work the method over at 2000 but have little opportunity for that distance and testing conditions. Hope this helped.
Shawn Carlock

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Old 02-18-2008, 10:52 PM
Silver Member
Join Date: Jan 2008
Posts: 412
Word to the wise, this kind of talk will not be well recieved in some circles, I have had people get red faced and want to fight over this subject. So be careful where you start spouting off about this stuff. When sharing this with folks who dont want to listen be especially watchful of the body language. As agitation sets in they will abruptly walk away from you and seek out like minded individuals a short distance away. They then will return with reinforcements and reengage in the discussion. There will usually be one individual leading the charge against this blasphemy of shooterdom. He will be surrounded by a pack of lap-dogs calling out encouragement to him as he calls your sanity,ability,patriotism and ancestory into question. This is usually a good time to get the --- out of Dodge unless your personality is like mine,in that case it is OK to stay for one last jab, work them into a feeding frenzy then try to escape with your equipment and health. Cheers.
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Old 02-19-2008, 09:41 AM
Join Date: Jun 2007
Location: North Idaho
Posts: 1,822
That is why I soley hang out here. If someone doesn't agree with my version how to do things, they still listen because the vast majaority of members here including myself are real shooters and don't figure they already know everything. I have learned a good number of things from this site as well, usually small things that just never occured to me until I read them and went "that sounds about right, I better try it out".
Shawn Carlock

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Old 02-19-2008, 08:59 PM
Platinum Member
Join Date: Feb 2004
Location: Pennsyltucky
Posts: 2,623
i was a little frustrated last summer with bullets impacting a little right at longer distances but i chalked it up to the fact that as you crank your turrets up and down, you're actually moving the crosshairs sideways also. i understand Sightron has eliminated this and maybe others have also. but most scopes have a round tube that the turrets push against and as you get farther from the 3:00 contact position, they actually move sideways. i've always felt the scope internals were my problem but after reading this info i'm convinced the SD might be the problem. reason i say that is because i have my scope canted so it's in the middle of it's adjustment in the 700-1000 range where i want the most "forgiveness of error"

i sent my scope to Cecil Tucker and he confirmed this with me in one of our conversations.
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Old 02-19-2008, 09:57 PM
Silver Member
Join Date: Jan 2006
Location: Skagit Valley WASHINGTON
Posts: 160
Shawn, you've been a huge help to me and a couple of buddies over here in WA setting up our Senderos in 300 Ultra and we appreciate it greatly. We've seen exactly what you are talking about at 1000 in terms of it varying from gun to gun. Same gun model, bullet, load, rest, day, and conditions, his gun will shoot with about twice as much spin drift as mine. I am starting to think of it as playing golf with my dad. He plays a huge slice (left to right) and I normally turn the ball just slightly left to right, but he has years of learning how to adjust for it and he just lays it out in the middle of the fairway more often than not. My question is with a wind coming from 9 O'Clock, will it have more effect on a bullet with more spin drift like it does with a golf ball? Will a wind from 3 O'Clock have less effect on the bullet with more spind drift because it is already heading into the wind?
Bill C
Ignorance is bliss...unless you're aware of it.
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