Home Forums Hunting & Shooting Ask The Gunwriters B.C question in regard to wind drift and trajectory

Here's an equation... not too nice, though.

Deflection in inches = 22.68 -149.2*BC^2 -0.00000107*(MV in FPS)^2 +0.0001505*(Range in yards)^2

The good news is that it works from 850 FPS to 3600 FPS, and from 65 yards to 750 yards. The bad news is that it will get you within about a foot.

Model was done at 4500 ft elevation, 65 F, 24.98 mm Hg, and RH 50%. Crosswind was assumed to be 10 mph.

It should be possible to do a much better model by tightening up the inputs so we're not trying to cover everything from soup to nuts. That will cost someone a box of bullets!

This thing called "drift" confuses so many in the shooting community it sometimes baffles me. It is not that complicated really, but there be a lot of folks come to the discussion with misconceptions defended with righteous certainty.

Glad to see some learning going on here in a civil fashion. Bullshooter, that was a very excellent post and on point.

Provided the bullets have the same form factor, if two bullets of different weights leave the muzzle with identical KE, the heavier bullet (the one with the higher BC) will ALWAYS experience less wind drift.

Really? What a revelation.

Thank you, Captain Obvious.

Thats got to be the funnest answer given . the bullet is spinning it does not have a sterring wheel or any traction to support it so it can turn to face the wind. if it did do that it would then change direction and hit the target an the winward side instead of the leeward where all bullets hit going through a cross wind. so you are full of chit if you beleive that.

Hubert, What Hawkins said is exactly correct.

When a bullet is fired out of a barrel, it has a forward velocity and it is spinning. Sufficient spin allows the bullet to remain stable in flight and by stable I mean the bullet keeps its forward flight, point on. If the bullet was not stable it would tumble and fly erratically; you no doubt know about keyholing and other such things.

Now, having this spin-stabilized bullet fly point on, does not mean that the bullet is pointed straight along the line of sight. Spin-stabilized bullet flight means the bullet is pointed straight into the incoming air just like one of those weather vane things you see on steeples, masts and so on.

When a bullet travels at let's say 3000 FPS and it encounters a crosswind of 20 MPH, the airflow changes because of the additional crosswind component. The bullet will turn ever so slightly to remain stabilized in the now-changed airflow. This is the effect of spin-induced stability on bullets, arrows, etc. The addition of the crosswind component is minimal to the overall airflow and the angle from the line of sight will be very small, less than a degree.

There is a trigonometric function called arctangent that you can use to calculate the new vector: arctangent(v2/v1) where v2 is the crosswind velocity and v1 is the forward velocity. So for out example, X=atan(29.4/3000) or .56 degrees. You can do this in Excel: =DEGREES(ATAN(((ws*5280)/3600)/3000)), where ws is the wind speed The result will be in degrees.

As you can see a 20MPH wind, which is pretty stiff, will produce a very small change in the bullet angle compared to its line of sight trajectory, but as forward velocity diminishes, this angle will become more pronounced, provided the crosswind remains the same.

This spins stabilized effect keeping the bullet pointed into the airflow is also the reason why bullets do not keep their noses up when they start coming down after reaching their maximum height in their trajectory.

And before you ask, crosswinds coming from various directions during the flight of the bullet will case the bullet point in various directions, depending on the airflow. And finally, having the bullet fly at angle to the line of sight does not increase its drag since it always stays pointed into the airflow.

I hope this helps.

Hubert, don't be full of chit. Take heed of FTR's words for he speaks truth. Hawkins too.

Your thoughts in the preceding post gives the impression your perspective trends toward powered flight, ignores various paradigms of flat fire ballistics, spin stability to include gyroscopic precession and most pointedly, aerodynamic drag.

Older Sierra reloading manuals contain a section explaining how exterior ballistics are calculated. Here's an image of the relevant section from the 1989 manual.



I have underlined the key concept of lag time, which is the difference between the Time Of Flight (TOF) in air and the TOF in a vacuum. You can think of this as how good a grip the wind has on the bullet.

Here are two examples of wind drift both using a bullet with a G1 BC of 0.500 and a 10 MPH crosswind.



As you can see this example contradicts some of the explanations and rules of thumb others have offered, but it's the numbers you'll get from any good exterior ballistics app, and if you work it out you'll see it follows the lag time formula.

So how can a bullet fired at 1000 fps have less wind drift at 500 yards than the same bullet fired at 2300 fps? Hint: BC is not a physical constant.

I went to JBM and used your figures of G1 BC .500, 10 MPH crosswind, and the two velocities of 1000fps and 2300fps. Since you did not specify a caliber and a bullet weight, I used .308 and 150 respectively for both velocities.

1000FPS 2300FPS
100: 1.3 0.9
200: 5.2 3.8
300: 11.4 9.0
400: 20.0 16.6
500: 30.8 26.9

So, in answer to your question. The slower bullet with the same BC has more drift (not less) that the faster bullet with the same BC. I submit to you that your table is flawed and should be discarded.

We know lag time is the important factor in wind drift, as I explained earlier and lag time is a direct function of BC. So one wold expect the same drift for the same BC bullet irrespective of the muzzle velocity.

And one would be wrong to do so. In fact lag time and wind deflection are a function of muzzle velocity and BC. I will also add that other things have to be equal, such as distance and atmospheric conditions.

So, if everything is equal, including the BC, why is the slower bullet experiencing more wind deflection than the faster bullet? The answer is that the slower bullet has a longer lag time. The reason that faster bullets have correspondingly less lag time is that higher velocities compress the flight timeline so that the VToF and the lag times are compressed. Less lag time, less drift. Also, bullets have lower drag coefficients at higher velocities.


Edited to add: The above chart was produced using JBM Trafectory (drift), from which I then drew the wrong conclusions. The regular JBM chart does not show a difference between varying velocities for the same BC. See my post later down for further explanation and discussion. Thanks to MacLorry for pointing out the discrepancy.

I don't know how you got your numbers from JBM, but here are my results showing the input values.



JBM does a bit more rounding than the app I used, yet it got 24.8 inches at 500 yards for both the 1000 fps and the 2300 fps loads. Using 15 digits for the TOF value in the calculations and the 1000 fps shot gets 24.70 inches at 500 yards.

BTW, for a given BC, bullet caliber and weight have no effect on the wind drift calculation. Don't believe me, change just those two values on JBM and you'll see you get the same wind drift results as before.

You have it backwards, BC is a factor in lag time and wind drift is a direct function of lag time and crosswind speed. Want proof? Just look at the following JBM example.



It's the same as the two from my prior post but the MV is now 2000 fps, which gives 30.1 inches of wind drift at 500 yards compared to 24.8 inches for the same bullet if launched at 1000 fps or 2300 fps.

I can explain why this is, but only if you agree my JBM examples are correct.

BTW, using Ballistic Explorer's Explore tool I easily found that the the peak 500 yard wind drift of 37.7 inches for the G1 BC of 0.500 bullet in a 10 mph crosswind occurs with a MV of 1463 fps. Bullets going faster or slow have less wind drift at 500 yards. How long would it take you to figure that out on JBM?

What is happening is that the bullet turns away from the path
and into the wind. Now part of the drag is down wind which
causes the drift down wind. Drift is not due to the "wind
blowing on the side of the bullet". It is not a function od
the time of travel. The reason it is a function of velocity
lost is that the BC is derrived from "Delay" functions.
People such a P.O.Ackley, and Elmer Keith did not understand
that.

Yes, as the bullet turns into the wind, the entire wind component is acting on the point of the bullet and because of the small crosswind vector, the bullet is being pushed away from the line of sight.

Now, if the bullet were completely slippery and did not lose any forward velocity, it would also not be influenced by the very small crosswind vector. Alas, that is not the case, but that is exactly where the BC of the bullet has a direct effect on its wind deviation analogous to its loss of forward velocity. If a bullet had no loss of forward velocity, it would also not be affected by a crosswind. Conversely, the more quickly a bullet loses forward velocity, the more it is affected by the crosswind component. And that's where lag time comes in; if there was no loss of forward velocity, there would be no lag time.

Whatever way the bullet turns, tilts, or twists, the wind drift formula from the Sierra reloading manual exactly calculates the wind drift. For the examples I have given the lag time peaks at around 1465 fps and so does wind drift. Fire the same bullet faster or slower and you get less wind drift at 500 yards. Note, however, that's not the case at other ranges, but for each range there is a MV that produces the peak lag time, and thus, the peak wind drift.

Finding the MV that produces peak wind drift for a given bullet and range is easy if you have the right software. It's a velocity I like to stay away from.

I will take a slower, high B.C. bullet 10 times out of 10 when shooting distance.

Low B.C. bullets suck balls in the wind.


Travis

Ahhh, now this is starting to make some sense. Thanks for that explanation.

This whole thread sucks. All this input from folks like Denton and FTR.

Where the heck is some more usable data from Hubert? Man I'm still waiting.

No offense, however, it seems that the maximum wind drift occurring around that velocity is more likely an artifact arising from some implicit assumption built into the formula / software. Running JBM ballistic calc for a 162 7mm AMax from the bullet catalog, and other default assumptions, it shows wind drift decreasing from the software minimum muzzle velocity of 500 fps to around 900 fps. The wind drift then increases from 1000 fps to about 1500 fps. After 1500 fps muzzle velocity the software shows wind drift decreasing as velocity increases (as one would expect) up to the software max of 4500 fps. It would be surprising if the increasing wind drift as muzzle velocity increases from 900 to 1500 fps actually corresponds to a real physical phenomena.

Well, it does correspond to a phenomenon. It's the high drag of the transonic velocity range. Case in point is the difference in deflection between subsonic and HV .22 RF ammo. Subsonic wins that race.

I don't believe the attempts to compare same BC bullets at widely disparate velocity is likely to be a rational endeavor, especially with sub and supersonic velocity if for no other reason than the dynamics of drag are quite different. BC is variable with velocity and the assumption that the two bullets will function with the same drag values over the course of flight is highly suspect.

Actually it has more to do with the bullet passing through or approaching the transonic range where drag changes faster than at any other velocity range.

When you select a bullet from the library the JBM software doesn't show you what BC it's using, so it might be wrong as you have no means of comparing it to the manufacturer's value. According to Hornady the 0.284 (7mm) 162 A-Max has a G1 BC of 0.625. If you enter that value into JBM and select "None" for the library you'll get different 500 yard wind drift and velocity values than when "Hornady, 0.284 cal, 162 gr, A-Max" is selected. Thus, the library value for this bullet's BC is not 0.625 and who knows if it's a mistake or they got an old value or something.

Finding the peak wind drift velocity using JBM or most other apps is a tedious error prone job which is likely why you got the results you did. Using Ballistic Explorer's Explore display I just move a slider to change the velocity while watching the wind drift values and graph, so it's easy to find the peak wind drift velocity. For the Hornady 0.284 (7mm) 162 A-Max with a G1 BC of 0.625, standard atmosphere, 10 MPH cross wind I swept from 500 fps to 5900 fps and found that the maximum wind drift at 500 yards occurs at 1430 fps, for 300 yards the maximum wind drift occurs at 1375 fps, for 800 yards it's 1526 fps. Easy to do when you have the right tool.

The purpose of this experiment was to disprove some of the concepts and rules of thumb being promoted, not to suggest there's a practical application. The lag time formula I posted is based on sound physics and has been in use for decades. It fully explains all the examples I posted.