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 November 29, 2012, 10:26 AM #51 Wyoredman Senior Member   Join Date: September 6, 2011 Location: Wyoming Posts: 1,320 Old Roper, Wheatland, FE Warren, Laramie, I-80 corridor, Rock Springs! Man you have lived in all the best WYoming has to offer...as far as wind goes! __________________ Go Pokes! Go Rams!
 December 1, 2012, 10:49 PM #53 tobnpr Senior Member   Join Date: August 1, 2010 Location: Tampa Bay Posts: 4,176 I dunno... Maybe I'm oversimplifying it... Gravity's effect/bullet drop on the bullet is linear as we all know...Newton... But the reason wind drift is not, is simply because as the bullet loses velocity, the lateral movement increases exponentially per unit of distance traveled simply because it's being "moved" or "pushed" the same distance per unit of flight time, (call it seconds, if you want) but it's covering less and less distance (per second) as it flies downrange so the relationship becomes exponential. I think.
December 1, 2012, 11:44 PM   #54
B.L.E.
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Quote:
 I dunno... Maybe I'm oversimplifying it... Gravity's effect/bullet drop on the bullet is linear as we all know...Newton...
Gravity accelerates stuff that falls. The distance fallen in ft = 1/2 * 32.2 * T^2 where T = seconds.
If the bullet doesn't slow down, you can expect 4X the drop at 200 yards as at 100 yards.

Exponential functions result in a curved path, like a bullet travels.
Linear functions result in a straight path, that's why we call them "linear".

 December 1, 2012, 11:52 PM #55 Brian Pfleuger Moderator Emeritus   Join Date: June 25, 2008 Location: Western Colorado, finally. Posts: 19,100 The reason wind drift isn't linear is because the speed of the wind is being added to the bullet, sideways. If the wind hit the bullet at 25mph and then completely stopped, the bullet would still be sliding sideways under the momentum imparted by the wind. Basic physics. An object in motion tends to stay in motion. The bullet begins to pick up its own horizontal speed. If the bullet wasn't picking up horizontal speed of its own, drift would be linear. But the bullet is picking up speed, it's accelerating. That's why the path is curved.
 December 2, 2012, 12:13 AM #56 B.L.E. Senior Member   Join Date: December 20, 2008 Location: Somewhere on the Southern shore of Lake Travis, TX Posts: 2,371 Right, the crosswind accelerates a bullet sideways. If the bullet actually kept up with the crosswind, a 10 mph (14.667 fps) crosswind would displace that .17HMR bullet 57 inches instead of 17 inches during the .324 seconds it took that bullet to go 200 yards. How 'late' a bullet is getting to the target is called the lag time and the moving shooter and target in a calm situation I detailed shows why wind drift is proportional to lag time, not time of flight. That 220 grain .30 caliber bullet with a muzzle velocity of only 1040 fps took nearly twice a long to reach the target, yet it drifted a whole lot less than that .17 caliber bullet. BTW, the drifts that my "moving target and shooter" model estimated agree with the wind drifts that ballistics calculators spit out in case no body got that.
December 2, 2012, 11:05 AM   #57
tobnpr
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Quote:
 Exponential functions result in a curved path, like a bullet travels. Linear functions result in a straight path, that's why we call them "linear".
Thanks for correcting me. I knew what I meant to say, but stated it incorrectly. What I meant to say was that bullet drop stays the same, regardless of wind velocity.

But isn't it correct that the "curved" horizontal trajectory (drift) of a bullet is due to the fact that it decelerates as it travels downrange?

IF a bullet traveled at a constant rate of speed, the flight path would be a straight line- albeit at a sharp angle to the target to hit point of aim if there were a stiff full-value wind, no?

 December 2, 2012, 11:37 AM #58 Brian Pfleuger Moderator Emeritus   Join Date: June 25, 2008 Location: Western Colorado, finally. Posts: 19,100 If the bullet could travel far enough, it would eventually be "sliding" sideways at the same speed as the wind. From that point on, the drift would be linear, more or less, depending on which variables you ignore for simplicities sake. It wouldn't quite be linear because it would be slowing in the forward direction with a constant sideways slide but it would be a lot closer to linear than it was during the time that the wind was accelerating it.
December 2, 2012, 05:35 PM   #59
tpcollins
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B.L.E. said

Quote:
 Actually, wind drift is exponential with distance in a constant wind. Here's the calculation of a standard velocity .22 long rifle bullet in a 10 mph cross wind. 25 yards 0.3 inches 50 yards 1.3 inches 100 yards 5.1 inches 200 yards 19 inches It's linear to wind strength, exponential to distance.

I agree - that's what my programs indicate.
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 December 3, 2012, 07:23 AM #60 B.L.E. Senior Member   Join Date: December 20, 2008 Location: Somewhere on the Southern shore of Lake Travis, TX Posts: 2,371 A bullet traveling through the air never feels a side wind hitting it. If you are traveling at 1000 mph, what you experience is 1000 mph wind hitting you from the front. The natural wind of the air you are moving through adds, subtracts, or shifts the angle that this wind hits you by a fraction of a degree. This "apparent wind" slows you down in the direction that the apparent wind hits you so if its direction shifts by a fraction of a degree, you slow down in that direction instead of the direction of your ground path. Think about it this way and it becomes obvious why it's how much you slow down and not the time of flight that determines your course deviation. As a bullet slows down, that wind angle also gets bigger, going from a small fraction of a degree to maybe a degree or so. The fact that slow bullets experience a larger headwind angle shift than fast bullets do explains why slow bullets drift more when the decelerations are equal.
 December 3, 2012, 08:41 AM #61 Bart B. Senior Member   Join Date: February 15, 2009 Posts: 6,318 B.L.E., it is the time of flight between two points in the bullets trajectory that determines how far from the line of sight the bullet drifts sideways. If you run a decent ballistics software program for a given bullet at a given muzzle velocity with its range increments set to 5 yards and maximum range at 1000 yards, it's easy to find out why. Find several range bands with 1/10th second time of flights. Those closer to the muzzle will be longer than those nearer the target. See how much drift the bullet has in each range band. That'll show you how much the bullet drifts at right angles to the line of sight for each 1/10th second of time of flight. This is what I think's reality. Gonna run Berger's software on a 30 caliber 150-gr. bullet with a .450 BC leaving at 2700 fps then check its drift for each 1/10th second of flight to 1000 yards. I'll post the results but it's gonna be a while before they're up on this thread. Gonna be gone for a couple of weeks and internetting ain't all that great via satellite feeds. Last edited by Bart B.; December 4, 2012 at 07:26 AM.
December 3, 2012, 09:00 AM   #62
Brian Pfleuger
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Quote:
 Originally Posted by B.L.E. A bullet traveling through the air never feels a side wind hitting it. If you are traveling at 1000 mph, what you experience is 1000 mph wind hitting you from the front. The natural wind of the air you are moving through adds, subtracts, or shifts the angle that this wind hits you by a fraction of a degree. This "apparent wind" slows you down in the direction that the apparent wind hits you so if its direction shifts by a fraction of a degree, you slow down in that direction instead of the direction of your ground path. Think about it this way and it becomes obvious why it's how much you slow down and not the time of flight that determines your course deviation. As a bullet slows down, that wind angle also gets bigger, going from a small fraction of a degree to maybe a degree or so. The fact that slow bullets experience a larger headwind angle shift than fast bullets do explains why slow bullets drift more when the decelerations are equal.
Well, sort of.

The bullet is not being "slowed down" by the wind. There are two vectors, friction and wind. Neither cares that the other exists, though the effect is as you describe.

It's actually not any different than considering the forces separately though. The bullet doesn't care that its going forward at 3,000 fps or hanging stationary in gravity free air. If the wind blew it would have the same effect. (Ignoring other forces for simplicity)

 December 3, 2012, 10:51 AM #63 tobnpr Senior Member   Join Date: August 1, 2010 Location: Tampa Bay Posts: 4,176 Exactly...that's what I'm saying. It's simply the fact that a bullet's velocity slows- exponentially- as it goes downrange. The higher the ballistic coefficient, the more velocity it retains, and the less it is affected by wind drift, compared to bullets with lesser BC's...and the "straighter" (meaning less curve) the line of flight. Fairly simple concept...
December 4, 2012, 07:35 AM   #64
B.L.E.
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Quote:
 B.L.E., it is the time of flight between two points in the bullets trajectory that determines how far from the line of sight the bullet drifts sideways.
If that was strictly true, then the .300 Whisper (essentially a .223 necked up to .30 caliber) shooting that 220 grain Sierra hollow point boat tail match bullet with a BC of .608 with a muzzle velocity of 1040 fps should have drifted way more than the .17HMR shooting that 17 grain bullet with a BC of .125 at 2550 fps.

Time of flights at 200 yards
.300 Whisper .598 seconds
.17 HMR .324 seconds

Wind drift in 10 mph crosswind
.300 Whisper 3.8 inches
.17 HMR 15.7 inches

Lag time, the amount of extra time added to time of flight due to slowing down increases exponentially even if the velocity decay is linear.
To understand this, visualize driving 60 mph and someone going 80 mph passes you. Even though you don't decelerate, he keeps getting farther and farther ahead of you and the lag time between the two cars increases.

The only way for a bullet to stop the increase in lag time after being slowed down would be to somehow re-accelerate back to the original muzzle velocity and stay at that velocity, then the lag time would be constant from that point onward.

 December 4, 2012, 08:28 AM #65 Brian Pfleuger Moderator Emeritus   Join Date: June 25, 2008 Location: Western Colorado, finally. Posts: 19,100 It's the time of flight for a given bullet, not between two bullets. A given bullet will drift twice as far in 0.2 seconds as it will in 0.1 seconds.
 December 4, 2012, 09:21 AM #66 kraigwy Senior Member   Join Date: June 16, 2008 Location: Wyoming Posts: 10,518 We could probably eliminate all this conflict and confusion if we were to take our rifle and a given bullet to the range, shoot it in different conditions and at different ranges and SEE WHAT HAPPENS. Silly concept I know. __________________ Kraig Stuart CPT USAR Ret USAMU Sniper School Oct '78 Distinguished Rifle Badge 1071
December 4, 2012, 10:34 AM   #67
Brian Pfleuger
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Quote:
 Originally Posted by tobnpr It's simply the fact that a bullet's velocity slows- exponentially- as it goes downrange.
That would be a NEGATIVE exponent. The slower the bullet goes, the slower it slows down.
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 December 4, 2012, 04:18 PM #68 Erno86 Senior Member   Join Date: September 22, 2012 Location: Marriottsville, Maryland Posts: 1,259 A headwind will slow a bullet down, a tailwind will speed the bullet up a bit. When a bullet transitions from sonic to subsonic, is a time when a wind can have a significant effect on bullet deflection.
December 4, 2012, 04:45 PM   #69
Brian Pfleuger
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Quote:
 A headwind will slow a bullet down, a tailwind will speed the bullet up a bit.
A tail-wind doesn't speed up a bullet. The bullet is slowing from the millisecond that it is no longer under the influence of pressured gases in the barrel.

The effect of a tailwind or headwind is exactly the same as if the bullet had the same speed difference in stationary air.

In other words, a 3,000fps bullet with a 20fps headwind will slow at exactly the same rate as a 3,020fps bullet in stationary air.

The same bullet with a 20fps tailwind will slow at the same rate as a 2,980fps bullet in stationary air.

The bullet has no idea if it is moving or the air is moving around it.

Of course, the actual effect at any given instant is a bit complex, requiring calculus... as Δt approaches 0 and whatnot...
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December 4, 2012, 09:22 PM   #70
B.L.E.
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Quote:
 A tail-wind doesn't speed up a bullet. The bullet is slowing from the millisecond that it is no longer under the influence of pressured gases in the barrel. The effect of a tailwind or headwind is exactly the same as if the bullet had the same speed difference in stationary air. In other words, a 3,000fps bullet with a 20fps headwind will slow at exactly the same rate as a 3,020fps bullet in stationary air. The same bullet with a 20fps tailwind will slow at the same rate as a 2,980fps bullet in stationary air. The bullet has no idea if it is moving or the air is moving around it.
It's also the same if you are in calm air and shooting from a vehicle going 20 fps. If you shoot forward, the vehicle's speed is added to the bullet and if you shoot rearward, the vehicle's speed is subtracted from the bullet. If you are firing at a target that you are chasing and the target is going as fast as your vehicle, it's exactly like shooting into a 20 fps headwind, same drop, same target impact speed.

Come to think of it, when I shoot at a range, I don't even have to think about the fact that the air I'm shooting through is actually traveling about 860 mph. It just seems calm to me because I and the ground under me and the targets I shoot at are also going about 860 mph from the west to east as the earth turns.
All wind is relative. The bullet can't tell the difference between wind caused by its motion and wind caused by the weather.

December 5, 2012, 04:58 PM   #71
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Quote:
 Vertical Deflection --- Head winds tend to slow the bullet because of added air resistance and the resultant drag. This is why the point of impact is lower. Tail winds cause just the opposite, thus making point of impact higher. This is called vertical deflection and is not as pronounced as horizontal wind deflection. At 1,000 yards a .30-caliber match bullet will have about a 1/2 minute vertical deflection up with a 10-mph tail wind. With a 10-mph head wind, the deflection will be about 1/2 minute down. A half-value head or tail wind will be about 1/4 minute deflection. There isn't any set formula or system to help figure vertical deflection, and most shooters don't even consider it. Sierra Bullets' ballistic computer program gives all the vertical deflections, even with the oblique wind angles. If you want exact figures for all ranges and wind angles, this program is the only way to go.
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 December 5, 2012, 05:13 PM #72 Brian Pfleuger Moderator Emeritus   Join Date: June 25, 2008 Location: Western Colorado, finally. Posts: 19,100 Yes, but a tailwind doesn't "accelerate" (in the common meaning of the word, technically any change in speed is an "acceleration") the bullet. It causes it to lose speed more slowly. For instance, a bullet fired with a 20mph tailwind will decelerate as if it's muzzle velocity were 20mph slower than it really is. A bullet with a 20mph headwind will decelerate as if it's velocity were 20mph faster than it really is. __________________ Still happily answering to the call-sign Peetza. --- The problem, as you so eloquently put it, is choice. -The Architect ----- He is no fool who gives what he can not keep to gain what he can not lose. -Jim Eliott, paraphrasing Philip Henry.
 December 26, 2012, 11:05 AM #73 Bart B. Senior Member   Join Date: February 15, 2009 Posts: 6,318 As promised....with Berger's software; interesting stuff indeed. It's all based on a single wind speed at all trajectory heights above a horizontal line of sight. Concusions: In a crosswind, bullets move: * sideways faster as range increases. * downrange less for each 1/10th second flight time. __________________ US Navy Distinguished Marksman Badge 153 Former US Navy & Palma Rifle Team Member NRA High Power Master & Long Range High Master NRA Smallbore Prone Master
 December 26, 2012, 08:35 PM #74 1stmar Senior Member   Join Date: March 21, 2012 Location: Connecticut Posts: 1,982 That's a lot of drift between ~900-945 yards, 9" in 50 yards..
December 26, 2012, 10:19 PM   #75
B.L.E.
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Quote:
 That's a lot of drift between ~900-945 yards, 9" in 50 yards..
Even if the wind had been dead calm from 900 yards out to the target, it would still have drifted nearly 9 inches between 900 and 945 yards. The bullet travels a curved path in the wind, once it leaves the wind, it travels a straight path, but that straight path still diverges from the original path of the bullet.

It's like a bullet that got deflected one moa by hitting a twig right in front of the muzzle. The deflected bullet flies in a straight line but it misses the bullseye more and more the farther away the target is. One inch at 100 yards, two inches at 200 yard, 3 inches at 300 yards, etc, etc.

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