The Firing Line Forums

Go Back   The Firing Line Forums > The Hide > The Art of the Rifle: General

Reply
 
Thread Tools
Old November 29, 2012, 10:26 AM   #51
Wyoredman
Senior Member
 
Join Date: September 6, 2011
Location: Wyoming
Posts: 1,256
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!
Wyoredman is offline  
Old December 1, 2012, 06:29 PM   #52
B.L.E.
Senior Member
 
Join Date: December 20, 2008
Location: Somewhere on the Southern shore of Lake Travis, TX
Posts: 1,904
I have been doing some thinking about how to make wind drift more understandable.

When you are driving a car down the highway and you spit out of the window, it looks like the wind outside the car takes whatever you spat out and blows it backwards.
But there is no wind out there, the "wind" you feel when you stick your hand out the window is due to the motion of the car. What you really saw was the velocity of the spit, which started off going forward just as fast as the car was going, slowing down and your car, which was not slowing down, leaving it behind. That spit actually went in a straight line from a stationary reference point.

OK, instead of the air moving, let's assume the shooter and target is what's moving. We have a shooting station on the deck of a ship and a target on another ship 200 yards away and both ships are traveling parallel to each other in the same direction at 10 mph in a dead calm. The ship's speed makes this dead calm look like a 10 mph crosswind.
The bullet obviously travels a straight line because the wind is actually calm.
Because the target is moving, the shooter has to lead the target just like when shooting at flying birds with a shotgun.
But the shooter is also moving and that motion gives the bullet the lead needed to hit the target. Gunners will tell you that when you are strafing ground targets out of the side of a helicopter gunship, you have to lead those ground targets just as if the helicopter was stationary and the ground was moving under you.
So these two things cancel each other out, as long as the bullet does not arrive late. The later the bullet arrives, the more behind the target the bullet hits.

Now consider a .17 HMR with a muzzle velocity of 2550 fps.

At 2550 fps, it takes .235 seconds to travel 200 yards, but the bullet slows down, at 100 yards it's only going 1900 fps and at 200 yards, it's down to 1380 fps and as a result, it takes it .324 seconds to go 200 yards. Subtract .235 from .324 and we see that the bullet arives at the target .089 seconds late. Since 10 mph is 14.667 fps, 14.667fps X .089 seconds is 1.3 ft or 15.6 inches more that the target moved while waiting for the bullet to get there.

Now let's look at a Whisper 300 shooting a 220 grain hpbt with a BC of .608 with a muzzle velocity of 1040 fps.
At 1040 fps, it should take .577 seconds to reach that target.
But at 100 yards, this bullet has slowed to 1002 fps and at 200 yards it has slowed to 970 fps and as a result it took it .598 seconds to go 200 yards.
This bullet is .021 seconds late and .021 seconds is enough time for that moving target to go an extra 3.7 inches.

The more a bullet slows down during its flight, the later it gets to that target and the more it misses the bullseye of that moving target.

Now here comes something that's totally counterintuitive. Let's say the bullet speeds up during its flight. Now the bullet gets to the target early instead of late and instead of underleading this moving target, we are overleading the target and the bullet seems to drift against the wind instead of with it. Of course, bullets never speed up during flight unless you are shooting rocket assisted artillary rounds.
I dunno, maybe rocket propelled projectiles actually do climb the wind instead of drifting with it while the rockets are accelerating the projectile.
B.L.E. is offline  
Old December 1, 2012, 10:49 PM   #53
tobnpr
Senior Member
 
Join Date: August 1, 2010
Location: Tampa Bay
Posts: 2,964
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.
tobnpr is offline  
Old December 1, 2012, 11:44 PM   #54
B.L.E.
Senior Member
 
Join Date: December 20, 2008
Location: Somewhere on the Southern shore of Lake Travis, TX
Posts: 1,904
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".
B.L.E. is offline  
Old December 1, 2012, 11:52 PM   #55
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,789
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.
Brian Pfleuger is offline  
Old 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: 1,904
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.
B.L.E. is offline  
Old December 2, 2012, 11:05 AM   #57
tobnpr
Senior Member
 
Join Date: August 1, 2010
Location: Tampa Bay
Posts: 2,964
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?
tobnpr is offline  
Old December 2, 2012, 11:37 AM   #58
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,789
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.
Brian Pfleuger is offline  
Old December 2, 2012, 05:35 PM   #59
tpcollins
Senior Member
 
Join Date: February 18, 2009
Location: SE Michigan
Posts: 443
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.
__________________
What direction did that last shot at Kennedy come from?
tpcollins is offline  
Old 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: 1,904
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.
B.L.E. is offline  
Old December 3, 2012, 08:41 AM   #61
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,835
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.
Bart B. is offline  
Old December 3, 2012, 09:00 AM   #62
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,789
Quote:
Originally Posted by B.L.E. View Post
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)
Brian Pfleuger is offline  
Old December 3, 2012, 10:51 AM   #63
tobnpr
Senior Member
 
Join Date: August 1, 2010
Location: Tampa Bay
Posts: 2,964
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...
tobnpr is offline  
Old December 4, 2012, 07:35 AM   #64
B.L.E.
Senior Member
 
Join Date: December 20, 2008
Location: Somewhere on the Southern shore of Lake Travis, TX
Posts: 1,904
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.
B.L.E. is offline  
Old December 4, 2012, 08:28 AM   #65
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,789
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.
Brian Pfleuger is offline  
Old December 4, 2012, 09:21 AM   #66
kraigwy
Senior Member
 
Join Date: June 16, 2008
Location: Wyoming
Posts: 9,444
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
kraigwy is offline  
Old December 4, 2012, 10:34 AM   #67
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,789
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.
__________________
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.
Brian Pfleuger is offline  
Old December 4, 2012, 04:18 PM   #68
Erno86
Senior Member
 
Join Date: September 22, 2012
Location: Marriottsville, Maryland
Posts: 532
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.
Erno86 is offline  
Old December 4, 2012, 04:45 PM   #69
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,789
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...
__________________
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.
Brian Pfleuger is offline  
Old December 4, 2012, 09:22 PM   #70
B.L.E.
Senior Member
 
Join Date: December 20, 2008
Location: Somewhere on the Southern shore of Lake Travis, TX
Posts: 1,904
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.
B.L.E. is offline  
Old December 5, 2012, 04:58 PM   #71
Erno86
Senior Member
 
Join Date: September 22, 2012
Location: Marriottsville, Maryland
Posts: 532
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.
Quotes from the book: Dead On, Authors -- Tony M. Noblitt and Warren Gabrilska, Paladin Press
Erno86 is offline  
Old December 5, 2012, 05:13 PM   #72
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,789
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.
Brian Pfleuger is offline  
Old December 26, 2012, 11:05 AM   #73
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,835
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
Bart B. is offline  
Old December 26, 2012, 08:35 PM   #74
1stmar
Senior Member
 
Join Date: March 21, 2012
Location: Connecticut
Posts: 1,198
That's a lot of drift between ~900-945 yards, 9" in 50 yards..
1stmar is offline  
Old December 26, 2012, 10:19 PM   #75
B.L.E.
Senior Member
 
Join Date: December 20, 2008
Location: Somewhere on the Southern shore of Lake Travis, TX
Posts: 1,904
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.
B.L.E. is offline  
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 04:29 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2014 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent: thefiringline.com
Contact Us
Page generated in 0.14121 seconds with 9 queries