spin drift

[Buffalobob]

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.

 

[malcarjeb]

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.

 

[Shawn Carlock]

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.

 

[Augustus]

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.

 

[Shawn Carlock]

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".

 

[davewilson]

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.

 

[tjbill]

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?

 

[Augustus]

Dave, I use USO SN3 scopes fitted with the Horus reticle and use the holdovers exclusively, a USO rail level is fixed and used religously. The reticle is plumb and there are no internals used in any way. The projectile just drifts and I cant stop it. I just had to admit it, learn how much it affected the rig I am shooting, correct for it and forget it. I think internals could compound the problem if it has an internally canted reticle. Someone smarter than me needs to address that subject.

 

[Shawn Carlock]

TJBILL,

Good question. My belief is that you will have wind drift + spin drift to the right and wind drift - spin drift to the left. Without having a side impact wind tunnel to shoot in I don't know if you would be able to accurately determine if a side wind increases/decreases spin drift or not. I have not seen it but that doesn't mean it doesn't exist. At 1500 yards for instance in the calm, spin drift might be 12" for a given cartridge. Now add wind drift of say 60" for a total of 72". If there is additional spin drift from wind forces I believe it to be minor in effect maybe a couple of inches. I believe that any additional spin drift caused by wind effects would be so small a number that compensating for it would be nearly impossible. We are talking about less than a 1/4 moa. Since you did not have enought to worry about I'll give you one more. In you scenerio of a 9:00 wind of say 10 mph and a RH twist barrel, you will have the following factors to deal with:
bullet drop (raw drop)
wind drift
spin drift
elevation variance due to 9:00 wind and RH twist (it will be lower impact and you will need additional elevation)
Oh yea a couple of other little issues, are you up or downhill? God forbid that the wind is blowing over a terrain feature like a ridge or draw. Oh you say the wind is not blowing the same direction at your target? Blah, Blah, Blah. If you can make good first round hits in less than perfect conditions past 1000 yards, pat yourself on the back. Anymore when I here that Long Range Hunting is the "easy" way out I just shake my head at the uneducated. Glad you guys are getting setup for 08 early.

 

[Shawn Carlock]

Agustus,

You are right on the money with the issue of the reticule and turrets not being plum. If the reticule is not square with the turrets it can and will cause windage changes as you dial up or down. Everyone squares the reticle to the action/barrel but what really needs to happen for dial shooters is the turrets need to be square to the action / barrel. I have seen reticules out of square far enough to cause 6 moa windage at 1000 yards!!!! There is a word to describe scopes like that but they don't allow it here. Several issues inside the scope can cause slight windage variations. Spring tension against the erector tube is one, systems with split leaf single springs will start to show this after the spring shows fatigue and puts uneven pressure on the tube. Scopes with single springs pushing at 45 degrees opposite the turrets seldom have this happen. Ultimately you want to have this issue squared away before you start dialing in your rifle.

 

[Boss Hoss]

Shawn that is so true-----like having a race car with loose lug nuts!!! Shooting LR in the field or in competition you will not be sucessful if you equipment is not either good enough to do the job or it is not set up correctly.

 

[davewilson]

i'm not talking about the crosshairs not being plum. i'm talking about the turrets being the 2 points that affect the position of the tube holding the crosshairs. in a perfect world, the windage adjustment is at the 3:00 position and the verticle adjustment is at 12:00. the top one moves it up and down, the 3:00 moves it sideways.let's say the windage adjustment is contacting the crosshair tube at the 4:00 position and you move the crosshairs up. this also moves the windage adjustment more to the right at the same time without touching it. in effect, you've moved the place it touches the tube towards the 4:30 position, and because a spring is pushing on the opposite side,this allows the crosshairs to move sideways when you've only made a verticle adjustment.

am i explaining this good enough? anyone?

 

[Boss Hoss]

Yes---that is why we use the "Box Test"!!!!!!! Never had a NF, S&B PMII or a USO fail one yet..

 

[Augustus]

Dave, I guess this is the reason I use the holdovers, I grew up using cheap scopes that would never return to zero if moved then returned to the same setting. Also they would not move at all when adjusted then suddenly take a huge leap. Once I get my zero I dont touch the knobs, this is just a phobia of mine. I have done box tests with the USOs and they have been flawless. It still gives me the willys to crank the knobs. The owners manual that came with the USO scopes addressed the problem you speak of and indicated when you get to the outer capabilities of the adjustments the problems you are talking about can occur. Get a good sloped base and stay out of the danger zone if it worries you. I am coming to grips with my phobia and have recently started using the knobs for wind and spin when shooting at a small target with no reference for the Horus Reticle.