Bullet Drift From Wind

Discussion in 'Rifle Country' started by grogetr, Nov 15, 2015.

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  1. grogetr

    grogetr Member

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    I went today to do some shooting and wanted to see how it did from 100 to 300 yds. I have a ar 15 with 16 in barrel. I am using a 55 gn v max loaded with 24.5 gn of w748 loaded to 2.250". These shoot 2811fps. At 100 yd it was right on target with a 1.25" group. At 200 yd elevation was good but it was 4" to the right with a3.5 in group. At 300 it hit 8 to 10 in right not really a group at all. The wind was blowing around 10 mph left to right with some gusts higher. Is this normal results with this wind?
     
    Last edited: Nov 18, 2015
  2. ku4hx

    ku4hx Member

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    Yeah ... I've seen worse. Wind drift is one reason snipers have spotters and all sorts things to try and compensate for it.
     
  3. Kingcreek

    Kingcreek Member

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    Yep that's typical, especially with gusting cross wind.
     
  4. The Big D

    The Big D Member

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    That matches what JBM says the windage should be for that load per JBM. You probably sighted in about 1" of wind.
     
  5. browningguy

    browningguy Member

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    Yep, that's about right. If you want to shoot in windy conditions you need long heavy for caliber bullets. Now people will want to make it complicated with secant ogives and lots of math (which I can do by the way), but to keep it simple, in general heavy for caliber bullets will go through the wind better. For a .223 that starts around 69 grains, and 75-77 is better.
     
  6. Walkalong

    Walkalong Moderator Staff Member

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    50 and 55 Gr bullets can shoot awfully well in .223, definitely much better than the OP's results so far.
     
  7. GooseGestapo

    GooseGestapo Member

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    Yep, right in line with my experience!
     
  8. grogetr

    grogetr Member

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    Thanks for the replies. The wind didn't seem that hard so I didn't realize it would have that much effect. Weather Bug said ave 8mph with gust of 18 but there weather station is 10 mi from me so don't know accurate that really was. I want to use these for coyote hunting and was checking the yardage dots on the scope to see if they were close which for elevation they were. Around here I probably will never be able to take a shot that long but its good to know.

    Walkalong, I"m not the best shot around I have trouble holding still so yes I think in the right hands the groups would be better but I will keep practicing and hopefully get more comfortable.
     
  9. outlawjw

    outlawjw Member

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    The wind is an animal that only practice on windy days will help you to take it down.
    A wind meter & ballistics app together will get you close but because of longer distance & manipulation of wind direction by the terrain in between you & target .only practice will help you conquer the wind
     
  10. ontarget

    ontarget Member

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    The wind is definitely a factor and longer ranges amplify the effects. I have seen the range flags at 200-300-500 yards all blowing in a different direction at the same time. Good luck doping that....
     
  11. giggitygiggity

    giggitygiggity Member

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    Yes, that is normal. Try shooting at 500 yards with a 14.5" barrel AR and a strong cross wind. In order to hit a man-sized target, you'll need to aim 1-2 body lengths to the side. Shooting with wind makes things more challenging and fun. Wind is not an excuse for missing a target.
     
  12. Caliper_Mi

    Caliper_Mi Member

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    Did you mean four feet (4') right or four inches (4") to the right at 200?

    Good rule of thumb if you don't have a ballistic app at hand is 1moa per 100yd per 10mph of crosswind.
     
  13. grogetr

    grogetr Member

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    Yeh it was 4" at 200yds. Thanks for all the info, now I have an idea how much you need to allow for wind.
     
  14. Corn-Picker

    Corn-Picker Member

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    I hope this isn't too OT, but I have a question for the theoreticians and the guys who shoot in the wind; does a heavy for caliber bullet help with wind drift and if so why?

    Theoretically, I understand why a heavy for caliber bullet slows down less than a light for caliber bullet; the drag force only depends on the frontal (cross sectional) area of the bullet (which doesn't change if the caliber stays the same). As the bullet gets longer its mass increases, so the same drag force is operating on a larger mass and thus slows it down (negative acceleration) at a smaller rate.

    When I think about wind hitting the side of the bullet, if a heavy for caliber bullet is 1.75 times the mass of a light for caliber bullet, it's about 1.75 times as long, and thus the wind has 1.75 times the area to apply force -- it seems like the light and heavy bullets would have the same wind drift if they had the same time of flight. To me, intuitively, it seems that the lighter bullet would have less wind drift in cases of all but the longest range shots (because the lighter bullet generally reaches its target faster).

    Also, if you are a very serious shooter, is the g1 drag coefficient a useful tool for calculating wind drift, or do you use some other coefficient? When I look at the bullet that the g1 coefficient was based on and compare it to a heavy for caliber 223 bullet it's like comparing a cannon ball to a canoe. I understand why g1 would be useful for calculating drop (because you adjust g1 until it fits the drop you observe), but it seems that g1 would underpredict wind drift in any bullet you would shoot from a shoulder mounted weapon.

    Thanks everyone, I learn a lot on this board and appreciate your input. There aren't many long range shots (or much wind) where I hunt, so I've never had to learn how to shoot in the wind.
     
  15. The Big D

    The Big D Member

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    Corn-Picker,

    Interesting question. The ballistic calcs consistently say that heavy-for-caliber loads have not only better energy retention due to BC, but better wind performance at least in the calibers I shoot. But now that you bring it up, I don't really understand why.
     
  16. Bart B.

    Bart B. Member

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    Wind facts.....

    A given crosswind speed close to the rifle has more effect on downrange bullet drift than that same crosswind near the target.

    Wind above the line of sight is faster than in the line of sight; how much depends on the terrain. Smooth, open, flat land has lower cross winds at the bullet's maximum ordinate than terrain with low bushes and short trees.
     
  17. giggitygiggity

    giggitygiggity Member

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    This is not an accurate statement. Wind at the target has more effect on a bullet than wind closer to the rifle's muzzle. As the bullet flies, it loses speed and is more susceptible to bullet drift closer to the target where it has less speed to cut through the wind.
     
  18. taliv

    taliv Moderator Staff Member

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    not sure why that debate has lasted so long. seems like it would be pretty easy to get a calculator with multiple wind zones and have a 10mph right to left in zone 1 and left to right in zone 2 and see whether it tells you to hold left or right

    i can't think of a way to do that in something like jbm
     
  19. LRShooting

    LRShooting Member

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    Put it this way, my 308 at 600 yards with 41.7 gr 4064 and 175 smk needed 32 in of correction to combat the wind. The wind would some times slow down to 3 mph and I had to recorrect to 10 in from original zero. At 900 yards I had less then 4 in of vertical spread. I think with 41.9 gr, I had less then 2.5 in. My horizontal on the other hand varied 6 inches because of slight changes in wind direction.

    223 is going to be even worse with those low bc numbers.
     
  20. LRShooting

    LRShooting Member

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    I was going to mention this as well. That weight is part of the ballistic coefficient determination so more weight leaves you with a higher bc. Bc is really a combination of factors you would normally have to enter one by one but the manufacturer is usually nice enough to just give you a number to plug in with everything figured out.
     
  21. Corn-Picker

    Corn-Picker Member

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    But, the bullet doesn't stop drifting when the wind stops. If you have zero cross wind from 0-150 yards and a 10 mph cross wind from 150-300 yards, the bullet is only drifting for 150 yards. If there's a 10 mph cross wind from 0-150 yards the bullet drifts, and even if there's no wind from 150-300 yards the bullet will continue to drift because it's already moving in that direction.

    If you setup the problem with a 10 mph right to left wind from 0-150 yards and a 10 mph left to right wind from 150-300 yards then the answer is less clear, i can run som back of the envelope calculations when I have some time. Intuitively I'd side with Bart, but it's not obvious like it is in the former case.
     
  22. Walkalong

    Walkalong Moderator Staff Member

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    Actually, Bart is indeed correct.
     
  23. taliv

    taliv Moderator Staff Member

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    it's complicated. I think Bart's correct as well in some circumstances but not others.

    example:

    using a 168g SMK with 2650 fps MV (defaults for all other values in jbm)

    at 500 yards, the bullet has 26" of wind drift and is now only traveling 1717.8 fps

    if we then stop and recalculate based on a MV of 1717.8 fps

    at 500 yards (which would be 1000 yards if we added them together) we get 52.7" of wind drift and a MV of 1040 fps

    so the second 500 yards has almost exactly 2x as much drift as the first 500

    HOWEVER that doesn't take into account at least two issues and maybe more:
    1. max ordinate is going to be in the second half of the trajectory, and wind is typically much stronger way up in the air than it is at the muzzle.
    2. this is simplistic in that in the second calculation, it doesn't take into account the direction of travel of the bullet, which is not toward the target, but at an angle based on the wind from the first 500 yards
     
  24. Walkalong

    Walkalong Moderator Staff Member

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    Wind drift at the muzzle shows up exponentially at the target. It's partly a matter of geometry.

    The variable is how much wind where and what direction. But for a reasonably constant wind the effect nearer the muzzle is now magnified by any other movement farther downrange, not to mention the fact that going offline early is magnified more and more as it moves away from that spot.

    If you are shooting 500 yards and the wind is light to two hundred, medium to 400, and heavy to 500, then yes, it gets moved more further out.

    Shooting 200 yard Benchrest you would see lots of flags at 10 yards to 150ish yards, but rarely any further out. I never saw any farther out than 175, but everyone had one closer than 25 yards to the muzzle. I would put out four flags shooting at 100 yards and five flags shooting at 200 yards. Only one more.
     
  25. Bart B.

    Bart B. Member

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    I think what some folks are forgetting, is that when the wind stops pushing a bullet sideways, the bullet still keeps going sideways to the line of sight at that rate all the way to the target.

    If you use good ballistic software to see how many inches sideways per 10 yards of downrange travel the bullet is moving when the wind stops, you can easily calculate how much to the side it will strike in the wind direction when it finally gets to the target.

    Here's Sierra Bullets' software plotting wind drift through 1000 yards. Green line's the bullet's horizontal path with a 9 o'clock 10 mph cross wind for the first 333 yards; red line for the last 333 yards.

    07e4db87-25a3-4282-a466-45b9070c7068.png

    This assumes the wind speed is constant at all points in the bullet's trajectory. No ballistic software I know of adjusts for wind speed above the line of sight that varies with terrain.
     
    Last edited: Nov 19, 2015
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