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9 o'clock vs. 3 o'clock wind drifts different with same wind velocity???

 
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  #71  
Old 02-16-2012, 10:06 AM
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Re: 9 o'clock vs. 3 o'clock wind drifts different with same wind velocity???

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Originally Posted by paphil View Post
If anyone has a program to dope spin drift, I'd be curious to know what it is at 2000 yards!
At 2000 yards with my 338 LM and a Berger OTM 300 gr. with an altitude of 4100 and a MV of 2800 spin drift is 1.7 MOA. according to my shooter program which puts me pretty much on even at these distances.

With my 300 win and a 210 Berger @ 2995 fps. (which I have not shot this far) shooter says 1.9 MOA

Jeff
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  #72  
Old 02-16-2012, 10:07 AM
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Re: 9 o'clock vs. 3 o'clock wind drifts different with same wind velocity???

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Originally Posted by paphil View Post
I'm only interested in curing part of the problem in the 9 vs 3 oclock wind difference. By simply canting my scope a small amount and testing it to be about 3/4 moa , I no longer need to have a 3moa hold for one 2 mph wind and a 1 moa hold for wind in the opposite direction. Do it your own way, I have no problem with that. And Top gun, the spin drift does increase faster than the linear equations would suggest but the drop of the bullet is related to the velocity of gravity and since at 2000 yards , I'm at 93 moa, the canted allowance would be about 3.7 X the 1 moa for 1000 yards . a little over 74". If anyone has a program to dope spin drift, I'd be curious to know what it is at 2000 yards!
Paphil,
To work it out I would need to know the length of your bullet, the twist rate of your barrel, bullet weight, B.C. and your velocity to give you a figure.
Spin drift is a function of the above as well as time of flight and departure angle.
There are a few ways that spin drift is calculated on hand held phones and PDA's. One way is as a fraction of the standard US mil ball tables, another is by approximation calculations that simplify a lot of more complex models (As per "Shooter)". A third way is how Exbal does it by user input data from test shots but this gives a linear result as I understand it.
Its handy to know which way your program works for ELR shooting but not real important for out to 1K.
It can be measured with doplar radar.
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  #73  
Old 02-16-2012, 10:36 AM
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Re: 9 o'clock vs. 3 o'clock wind drifts different with same wind velocity???

Like I said earlier , what I'm doing is to help with the doping the wind. What is pretty generic at 1000 is not generic at 2000. When each degree temp change is 3 inches of verticle and 100 feet elevation is worth 6 inches of verticle, you better have a program to help! I'm not a scientist ,just want to help people shoot. I'm mainly concerned with hunting out to1000 and teaching the basics of what people need to know to make good shots. When you add angles to the shot, it gets even more complicated. I doubt the Canadian had any advanced math ability to shoot 2600 yards but he did it anyway! Be aware of the variables and you can correct for them. I do admire those of you who have the ability to calculate what you need to do to make long shots. But that said, I remember a similar post where a "scientist claimed that the only place to truely test spin drift would be in a vacuum! Da.... Without air there would be no spin drift! I don't know for sure but I would suppose there would be less drift at 10,000 feet than at sea level. Thinking is a good thing and if I get you thinking, someone will invent a better way than what I'm doing.
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  #74  
Old 02-16-2012, 10:56 AM
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Re: 9 o'clock vs. 3 o'clock wind drifts different with same wind velocity???

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Originally Posted by Topshot View Post
To work it out I would need to know the length of your bullet, the twist rate of your barrel, bullet weight, B.C. and your velocity to give you a figure.
Because all rifle bullet's BC's change with their speed through the air, don't you need different BC's for different speed/velocity bands?

I would think so, but I might be wrong.
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  #75  
Old 02-16-2012, 11:14 AM
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Re: 9 o'clock vs. 3 o'clock wind drifts different with same wind velocity???

I'm leaving CE alone until I know which side my bread is buttered on. S.D. and a canted scope work very well when set up at the longest expected yardage. When closer ranges are shot the poi traverses closer to the center of the optical radius center and then past center keeps going (traversing), The closer the distance the less the error and the more poi can be off to hit the same size target. If a person sets up at a closer range and then shoots long range the error gets larger with distance and the target gets smaller ( not really). Setting up at max range reduces error from S.D. Setting up at close ranges increases error. I have no Idea what a rifle set up for ranges that span from 100 yds. to 2000 yds would be, Its got to be a bunch though. The only thing that makes the craziness of ELR shooting possible for me is to "use a dedicated rifle" and if I were to try to shoot closer ranges with them it would be a monkey and football show. Even at that I have very firmly hit a brick wall at 1800 yds and most folks will get frustrated to the point it seems like a joke to them and quit playing, It is quite the adventure!
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  #76  
Old 02-16-2012, 11:44 AM
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Re: 9 o'clock vs. 3 o'clock wind drifts different with same wind velocity???

Guys this might not be of interest but I will toss it out there for what its worth since it was just yesteday and fresh on my mind.

In talking with a few other wolf hunters I ran into while hunting an area, we got to talking long range. They wanted to see a LR shot and I am always eager to send one. So a rock was located at 1554 yards. Rock size was basically 2moa x 2moa measured with my NXS RN-R2 reticle. With all eyes and glss on the rock I doped the scope and sent one. Wind was very very light and I didn't even add it to the dope as it was so light and the target was so large. I hit about 1/2 moa right and 1/2 moa low from point of aim. I sent another using a little hold to center and we didn't spot it, I think it hit center rock but cant be sure. So I went back to the center mass hold like in the first shot and sent another. It landed in the same 1/2 moa low and 1/2 right spot as the first.

Now my point here is this. As I recorded this info in my field log book I wondered why I was 1/2 x 1/2 moa off. I had not employed the CE function as I felt I was good to go with out it. So I re-ran the dope on "shooter" this time elmloyng CE and getting the azmuth. The CE effect for this shot was another .4 moa left. Plus I believe I didnt give my kestrel time to get a good temp reading and that could easily have been the 1/2 moa low. My kestrel is pretty slow at temp readings I need to find a better way for getting current temps faster.

Anyway you cut it Corriolis Effect would have put me closer on this shot.

End result is, they were impressed and are coming to some of the long range shoots I do this summer. So I may have gave them the bug.

Jeff
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  #77  
Old 02-16-2012, 11:59 AM
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Re: 9 o'clock vs. 3 o'clock wind drifts different with same wind velocity???

That is what I was saying about temperature and altitude changes! At 1500 yards, 1 degree is about 1 inch vertical so if you were off by 7 degrees that could explain some of it. Also , a 1/2 moa gun could miss by that much. At that range even hitting the rock is a great shot!
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