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

MacLorry, good catch. It also threw me for a loop for a little while, then I realised where I screwed up.

There are two trajectory functions at JBM, the regular one, which is the one you used and then showed and another one called (drift) which adds the spin drift of the bullet. Since I am a long range F-class competitor, that's the one that I used as it's in my favorites on the computer.

So, I went back to JBM and selected the original calculator and sure enoguh I go the same numbers as you did; no difference in drift using the same BC regardless of distance.

But that brought out another issue. Originally, I always figured same BC would experience same drift at same distances regardless of muzzle velocity, but in reading Bryan Litz's book back in 2010, he explained how that was not so. It took me a while to dig up his book yesterday but I found it last night and read the particular chapter again. I found his explanation in chapter 5 and read again conclusion #5 on page 80 of the book, "Applied Ballistics for Long Range Shooting", first edition. That conclusion reads: Increasing muzzle velocity reduces lag time for a given BC.

So, I had remembered well, but JBM does not show that. I will be communicating with Mr. Litz and asking him about this. I will let you know what I find out.

Again, thanks for pointing out my error.

Thanks. I realised that I had not properly taken my earlier discussion to its logical conclusion.

Next time you're driving and you reach 70mph or so, open the window and stick out you hand and see if you can feel the 10mph crosswind hitting your car on the side. The people who think a crosswind pushes a bullet sideways need to try that little test and then try to imagine going 20-30 times faster but with the same crosswind.

However, all these crosswind formulas and deflections are simply intellectual exercises because that almost never happens in reality and certainly not at longer distances. Wind varies in strength, direction and duration unpredictably and that's what confounds LR shooters.

However beyond just distance we also have trajectory and terrain features that come into play. Wind has gradients and blows at different speeds depending on the altitude of the bullet. So many things to keep in mind, it's a wonder we can even stay on paper sometimes.

It's not really your fault. Trying to use JBM or most other software to work out ballistics problems is a daunting and error prone task given their clunky interfaces. I have the advantage of using Dr. Oehler's Ballistic Explorer to explore ballistics, but it seems for most shooters free trumps good at least when it comes to software. Okay, I got Ballistic Explorer for free when I bought my 35P chronograph nearly a decade ago, but I'll defiantly buy it when (if) they quit giving me free upgrades.



Being you have the first edition of Litz's book take a look at the bulleted items on page 86 and you'll find this statement: "Lag time is the fundamental measure of merit for wind deflection." The heading of the bulleted items is "Practical take aways from the wind chapter", and in practical terms increasing velocity decreases lag time. What I demonstrated was the exception that occurs near the transonic velocity range. My purpose in doing so was to demonstrate that rather than BC or velocity, lag time is the fundamental measure of merit for wind deflection just as Litz says in his book. Practical take aways are great, but it's useful for shooters to know about the exceptions to the rules even if they have limited practical value.

I know stuff happens in the transonic area and not all of it is good. This is the reason why I work so hard to have my bullets arrive at the 1000 yard target above Mach 1.25. This is the start of the velocity envelope where stuff starts happening as the build-up to the transonic passage starts. So for all my intents and purposes the transonic area is not one that I worry about.

Currently, the main discussion we have on the line between matches is the effect of near wind Vs far wind. It was fun re-reading Litz's thoughts on that and they match my observations.

I recently discovered a way to quantify the effect of terrain features on the trajectory of the bullet, especially with regards to our range where we have firing lines at various distances. I found this at a kite-flying site, of all places. But when you think about it, wind does have a lot of influence on kites. So I am discussing this with my fellow shooters and we will see if we can predict verticals in light of this find.

Terrain effects? Surely you jest! Say hello to my nightmare!

I jest not.

From the picture you provided, my working hypothesis (not even a theory yet,) is that any wind coming from the left (from the camera's POV in your picture) would create some type of build-up and disturbance right in front of the berm on your right. This would be more important if you shot from the bench or off-hand as opposed to prone. If the terrain opens up past the copse of trees on that right berm, then you would have issues from wind coming from the right at that point.

I don't think you shoot over the berm in the middle so that would not affect the vertical in any way.

Though to be sure from one picture.

Was jesting about your jesting sir. Of course it is a factor. The day this picture was taken winds were 8-12 mph, variable from 8-11 o'clock. The berm left center is about 60 yards down range is about 10' high and the bigger berms are about 20-25' in height. So they are significant players when shooting the 100 meter line in the back. Mechanical turbulence occurs when airflow is diverted around terrain or buildings. It is the nemesis of shooters and pilots. Big issue in mountains and for LR shooters.

In the case of this particular range, there is also a berm on the left of same dimensions as the right and a 10 and 25 yard berm to the left out of frame. Crosswinds play havoc with precision shooting here and the primary influence presents as a standing vortex generated by the taller berms. This circumstance is not unlike large scale rotors associated with lenticular clouds in the Rockies. The link below provides discussion about air movement in such scales and structure.

http://en.wikipedia.org/wiki/Lenticular_cloud

I was jesting about your jesting about my jesting.... Oh, you get the drift. It's all in good fun.

This discussion about the wind is interesting....all of it is enough to make the less technically inclined develop a headache but I know it is important

Saturday I was at the 600 yard line;the range is tree-lined both sides,but more open at the butts.I had three windflags up on flag poles (that are permanent fixtures on my range)located at 500,200,and above the butts.

Winds (according to the weather reports on the radio,varied 10-20 miles an hour,and as I watched the flags,the wind was variable right to left,but the flags varied at the same time,with two pointing right,the other pointing left,and they changed by the second as the wind switched back and forth.

I was very confused about how to manage this,but also noticed the mirage,closer to the ground,was only leaning a little to the right from vertical.I figured most of the turbulence was up higher,decided not to allow for the wind at all,and held dead on instead of shading one side or the other.I waited until the flag at the butts settled down for each shot,but the 200 and 500 yard flags were still switching back and forth.

Target was a 6" orange dot;rifle was a 7mm Mashburn with 162 Amax;scope a 6X Leupold.The result was a 4" 3 shot group with the first two in about 1 1/2",at 9 o'clock,the third a bit lower at 7 o'clock,all three a little outside the edge of the dot.

This was mostly lucky guessing but I was impressed how that bullet cut through those conditions.

I hear you there! A couple of days ago I was out doing some steel smacking, and I was getting about .75MOA of drift, even though I was unable to detect any crosswind whatsoever, both by feel and by visual cues. The target was located across a decent funnel in the bottom of a field, and the terrain was funnelling the crosswind above the vegetation that was visible from my position. One of those days where I simply couldn't account for the wind drift, but I had a good guess at where it was coming from.

The thing that didn't make sense in my head started with denton's explanation above. It makes sense that the bullet points into the wind, continuing in the forward direction at a slanted angle, but what didn't make sense is that the drag in the forward direction would now be pushing on the body of the slanted bullet, pushing it into the wind. The error in that theory, is that there is no forward drag, only drag in the direction of the wind. The wind drift, as you mentioned, comes from the force of the wind and the consequent drag action on the point and ogive of the bullet, pushing the bullet in exactly the direction of the wind.

When I first started sail boating I was amazed at how the wind ran in steaks across open water miles away from any obstructions. You could see the affect of these fast moving streams of air on the water surface once you learned what to look for and we would head for them just like sailplane pilots fly from one thermal to another thermal. We weren't trying to sail anywhere, just go faster.

I read somewhere that judging the wind is where shooting goes from being a science to being an art.

I'm working on a load that's nearly immune to wind and gravity, but so far I haven't found a man portable 10 megawatt laser.

That doesn't explain why the calc shows greater wind drift at 1000 fps muzzle velocity vs. 900 fps. Both starting off at well under the given speed of sound of 1116.4 fps, and of course well under at 750 yards which is where I was comparing wind drift (and which figure I inadvertently failed to include in my initial post).



Actually it does display the BC from the library on the output. It shows 0.625 G1 (ASM) for the one I specified. Although, even if it were using the 'wrong' value, I do not see how that addresses the point regarding the anomalous wind drift results (i.e. greater wind drift for higher velocity).



While it was rather tedious, I do not see how that would impact the final result. I simply set the muzzle velocity to various values, starting at the min of 500 fps, using 100 fps increments. While I did not identify the exact 'max' as you've indicated, the methodology was adequate to identify the pattern of wind drift decreasing as velocity increased from 500 to about 900, then wind drift increasing as velocity increased from 900 to about 1500 and then decreasing again as velocity increased above 1500. Are you saying that the result obtained from the calc is incorrect?

When a bullet goes through the transonic range, it may not affect lag time all that much, but it certainly affects the effective BC while the bullet is in that unstable state. What would explain the added wind drift of a bullet that crosses that velocity threshold as compared to a bullet that starts off below it.

You're not going to believe this but you're both right. It depends on which bullet you select from the library. For any bullet listed in the library where you see the word (Litz), the BC value will not be displayed. I understand the Litz values are G7 BC values and they are his property. If you want to find out what these values are, you need to buy his book. I have switched over to G7 BC a few years ago after reading his book; he makes a real good case for using that value. I calculated the G7 BC value for my boutique bullets and I use that value, plugging it into JBM when I need it.

Discussions on this topic can be very complex, or simple if one is inclined to make it so. I like simple for the most part.

Falling back to the original question and from a practical perspective, there are two parts to the question and I think therein is found the confusion or misunderstanding so prevalent in the discussion. Drag v. Velocity. There are a lot of folks out there who do not understand either the complexities of spin stabilized flat fire ballistics, or the fundamental simplicity of drag vectors and how this relates to wind deflection.

Sitting in a parking lot waiting for my bride and pecking on the I-Phone ain't the path for this chat because I need to toss up a couple images which I cannot do at present.

I'll be back....

The bullet passing through the transonic range does explain what you are observing, I just didn't spell out all the details of how. With Ballistic Explorer you would find that wind drift at 500 yards decreases with velocity from 500 to 850 fps, then increases with velocity to 1426 fps, then decreases as velocity increases after that. So the opposite of what's expected occurs between 850 fps and 1426 fps. Guess what, the middle of that velocity range is 1138 fps, almost exactly the speed of sound.

It's not the bullet's speed that's important, but the speed of the air as it flows around the bullet. For example, a bullet with a 45 degree cone for a tip requires air to move along the surface of the tip to get out of the way of the bullet. From basic trig it can be shown that for every inch the bullet moves forward the air along its tip must move 1.414 inches to get to the edge of the bullet. Disregarding other factors, the air along the tip of such a bullet would hit the speed of sound when the bullet had a velocity of just 790 fps. The opposite happens at the tail of the bullet, and thus, for a bullet with a shape that matches the G1 standard projectile, the effect spans a velocity of from about 850 to 1426 fps. This is why it's called a transonic range.



Interesting and yet you have to have some valid BC value entered even if you select a bullet from the library, otherwise it doesn't get results. Also, if you select the A-Max that displays 0.625 G1 (ASM) and note the velocity at 500 yards and then select "None" for the bullet library and enter 0.625 G1 for the BC you'll get different results for velocity at 500 yards. I expect a G1 BC of 0.625 to produce the same results regardless of the bullet being selected from the library or entered manually. When that's not the case I suspect a software bug or some other artifact. After all, you were thinking what I was observing was some sort of artifact.



It has to do with human error rates. The more tedious a task the more likely a person is to make a mistake that leads to bad data and erroneous conclusions. While you entered about 45 velocities to span 500 to 5000 fps, I simply moved a slider control up and down while observing both the numeric values and a graph of wind drift. When I got to an interesting velocity I switched over to the fine slider control to do 1 fps increments. I got better numbers in a fraction of the time and so I explored other ranges and gave that data. I've also looked at the Berger 0.284 169gr VLD bullet with a G1 BC of 0.617 and a G7 BC of 0.316 to see if the transonic range differs with the drag function, and it does. Yes you can figure it on using JBM if you really want to spend the time to hack through the problems.

I started a topic on the G1 vs G7 drag functions last year that even sucked in Bryan Litz. Basically, if you want to optimally match trajectory (drop) you need to use the equal TOF to a range method to convert G1 BC values to G7. It's something Dr. Oehler documented in the July 2007 Shooting Times issue. Of course, there's only one ballistics program that I know of that includes that method, the one Ken sells on his website.

OK, so something 'different' occurs in the transonic range. However, I'm still not convinced that the software is accurately modelling it. For one thing, the effect noted (higher velocity resulting in higher rather than lower wind drift) only appears to occur for muzzle velocities in the transonic range. However, other velocities would have the projectile decelerating through the transonic range, especially when considering longer ranges. Yet that doesn't seem to appear, as once the muzzle velocity exceeds the transonic range, the wind drift appears to consistently decrease as velocity increases.





Requiring a value in the BC field appears to be some sort of system validity check. If you select a bullet from the library, it doesn't seem to matter what number is entered in the BC field. Not sure why you get a different result when manually entering a BC as opposed to using the library. Perhaps there is additional information regarding the specific bullet, not displayed, which factors into the calc vs. a 'generic' BC.




OK, however, other than the increased probability of error, you've not really disputed the figures I generated so I still don't see why it matters. If the figures quoted are correct, then how they were generated no longer matters. In fact, you've stated the same findings, that for muzzle velocities in the 'range' (which you narrowed down more precisely) higher velocity shows higher wind drift. Again, the fact that it appears to only apply to muzzle velocities leaves me to question whether or not it is being handled correctly.

My problem is that I am not a very good shot. I can input all the possible variables into programs and get an aswer as to where I'm supposed to hold the rifle and as the distance increases, I can't seem to hold it properly because the bullet doesn't go exactly where it's supposed to go. I'm sure it's my shooting because these programs can't be all wrong. I have even used my wind meter to get the value at the firing line, but somehow the bullet doesn't always go exactly where I aimed it at 1000 yards.

I'm still learning to shoot and it's a neverending quest, I fear.