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Effects of wind?

 
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  #8  
Old 08-30-2005, 09:18 AM
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Re: Effects of wind?

JB,
I noticed it with my 260Rem. I just ran the .7 BC 3000 fps, because I'm waiting for my new 7-270WSM to show up!
Even with 20mph head and tail wind, the computed difference is only .5MOA.
Usually you would never get that much difference in one day.
Where I shoot it is also more complicted because it is open behind us, a dip in the range between 200-400 yards, and there is a hill to the left of us.
We get all kinds of up/down drafts as well.
Nevertheless any decent long range cartridge shoots very flat from a geometry point of view.
I still say that the calcs are correct that a strong head wind will lower the POI.
The target rifle guys who told me this all shoot .308 with 155 grain bullets.
Anytime we are seeing enough to be important, there are probably up/down drafts involved as well.
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  #9  
Old 08-30-2005, 10:25 AM
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Re: Effects of wind?

[ QUOTE ]
JB,
I noticed it with my 260Rem. I just ran the .7 BC 3000 fps, because I'm waiting for my new 7-270WSM to show up!
Even with 20mph head and tail wind, the computed difference is only .5MOA.
Usually you would never get that much difference in one day.
Where I shoot it is also more complicted because it is open behind us, a dip in the range between 200-400 yards, and there is a hill to the left of us.
We get all kinds of up/down drafts as well.
Nevertheless any decent long range cartridge shoots very flat from a geometry point of view.
I still say that the calcs are correct that a strong head wind will lower the POI.
The target rifle guys who told me this all shoot .308 with 155 grain bullets.
Anytime we are seeing enough to be important, there are probably up/down drafts involved as well.

[/ QUOTE ]

You definately have some updrafts there then [img]/ubbthreads/images/graemlins/shocked.gif[/img]

the 260 and 308 info above came from a FLAT range, and the 6br was fired at williamsport 1KBR

the br rose almost 12" with a 15-20MPH head gust.

the 308 (168SMK) and 260(139scenar) rose 1.5-2.0moa with a seemingly constant 20MPH headwind.

I'll stick to my thinking, where you cant make up a hard and fast rule for every round and every range [img]/ubbthreads/images/graemlins/smile.gif[/img]

Do update me if you see anything different, and i'll do the same [img]/ubbthreads/images/graemlins/wink.gif[/img]

Cheers,
JB
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  #10  
Old 08-30-2005, 10:48 AM
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Re: Effects of wind?

Sounds good JB. Maybe I'll check McCoy's book tonight.
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  #11  
Old 08-31-2005, 08:52 AM
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Re: Effects of wind?

JB, I flipped through McCoy's book and found explanations that might fit both our observations.
He says that a pure head or tail wind (with no crosswind) lowers or raises (respectively) the trajectory by a small amount.
This agrees with the calcs I ran.

What could explain what you saw is this business of the vertical component from a cross wind.
If a righthand twist barrel is zeroed at in calm conditons, then wind from the left will push the bullets to the right and down.
Wind from the right will push the bullets left and up. If the wind slowly changed, the string of bullets would form a slanted line (about 20 deg for many bullets, twists etc), going from top left to bottom right.

So if you had wind from the left when you got zeroed, then the wind switched to a headwind, the new headwind would want to lower the trajectory (only a small amount). But the loss of the vertical component from the crosswind (which switched away) would be a bigger effect, and the net result would be a raised trajectory.
Were your headwind gusts switching from a crosswind from the left?
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  #12  
Old 08-31-2005, 10:36 AM
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Re: Effects of wind?

Rich -- at williamsport, anything is possible, and that is the prevailing direction, but i honestly dont remember. All i remember was my bud was centered up and down, then a head gust just as he shot and the bullet hit around 10" high

the crosswind though would have had to have well over 10" in the vert component to use that explaination, and i dont see that being possible.

JB
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  #13  
Old 08-31-2005, 11:14 AM
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Re: Effects of wind?

At 1000 yards a crosswind worth 3-4MOA (horizontal), would have about 10 inches of vertical. The angle of the line of shots on the target for different winds should be 15-20 degrees...
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  #14  
Old 08-31-2005, 11:20 AM
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Re: Effects of wind?

Alright I guess I should have thought about it and explained myself more. A headwind with the target perfectly(or very near) level with the shooter and the ground level from shooter to target will impact lower. A tailwind, higher. Now if you on high ground and shooting over a depression, valley, ravine, etc., a headwind will raise POI due to updrafts. Tailwind will lower.

In my expierence shooting 308's and 300win at LR, almost always a headwind will raise POI, a tailwind will lower POI. But at the "targets" I normaly shoot at I dont really worry abou it, as I haven't seen much more then 1/2MOA deflection either way.
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