Wind drift - theory vs reality

[Ballisticboy]

There are a couple of points which have been touched on in the last few posts. The easiest is the muzzle velocity. Lag time will be affected by the muzzle velocity as well as the drag and mass of the bullet. This is obvious since if we have a very slow high drag heavy bullet with the same BC as a very light low drag bullet then ultimately the lag time of the heavy bullet could be greater than the entire flight time of the light weight bullet. The down wind drift of some low speed projectiles can be shown to be roughly inversely proportional to BC multiplied by muzzle velocity. I do not know if this type of simple relationship can be applied to bullets, I have never tried it.
The second point concerns the bullets ability to turn into the wind. The equations given by a previous poster apply to the point mass model which assumes the bullet will instantaneously turn into the wind. As noted this will obviously not happen. The speed with which the bullet will turn will be a function of its gyroscopic stability factor, the higher the factor the slower the bullet will turn into the relative wind flow. If a bullet is so highly stable as to require a significant length of time to turn into relative airflow from a cross wind that bullet will also have problems correcting any other yaw angles or yaw rates and will thus tend to produce large group sizes. It is unlikely that any shooter would be using such a rifle bullet combination, particularly for ELR hence the point mass model is surprisingly accurate in predicting known wind conditions. However, if the wind conditions are extremely turbulent with a large number of changes in speed and direction between the shooter and the target, then it may be possible that there will be small differences due to the different yaw behaviour of the different bullets.
When it comes to shooting two rifles and bullets alongside each other there are other things to consider rather than just the published BC values. The drag law for a particular bullet is a function not only of the bullet shape and speed but also of the complete shooting system including the rifle and the shooter. Slight changes to any one component in the shooting system can and will produce changes to the bullet drag. Finding two bullets, a heavy one and a light one, combined with two rifles giving the same muzzle velocities and two shooters which produce identical bullet retardation at all speeds is going to be challenging to say the least.

 

[Papa Charlie]

I don't believe that a test using two different rifles and two different shooter will result in true evaluation. Every barrel is different, more or less drag, sharper or smoother rifling, etc.
If we were to assume that you could find two barrels that will have almost neglegent differences you could set up two mounted test guns that are tied together. This would insure that you are aiming both the same. It would be easy enough to fire both at the same time and measure the difference at the target. But this would not tell you the strengths of the wind at different points along the path at the time the bullet travels through it.
The only way to do this would be to utilize a test chamber where the wind and its direction could be controlled and measured. To fire the rounds, you would have to use a single test gun that is mounted to remove the shooter from the equation and to ensure that different barrels would not have an effect on the outcome.

 

[dcali]

The drag law for a particular bullet is a function not only of the bullet shape and speed but also of the complete shooting system including the rifle and the shooter. Slight changes to any one component in the shooting system can and will produce changes to the bullet drag.

I see how the rifle can impact the drag characteristics, but if you assume equal muzzle velocity, what is the mechanism by which a shooter can influence drag? Minor variations in an initial lateral velocity due to differences in recoil control?

 

[dcali]

IMO - this topic is at the very heart of another current thread regarding the wind drift of two bullets of markedly different weight, but fairly similar BCs. More specifically, a relatively new 131 gr .257 cal Blackjack bullet is being offered with an advertised G7 BC of 0.345. In theory, this bullet would exhibit almost identical wind drift at 1000 yd when launched with a muzzle velocity of ~2940 fps as compared with a 180 Hybrid at 2820 fps. As you might imagine, many naysayers have chimed in with purely anecdotal information suggesting that lighter bullets do not always behave in terms of wind drift as may be predicted by ballistic calculators.

The problem with using anecdotal information to support the argument that lighter bullets behave aberrantly with regard to "predicted" wind drift as compared to heavier bullets of comparable BC is that rarely, if ever, are the bullets compared side-by-side in an empirical manner. There are a few different ways you could conduct reasonable tests to assess the wind drift of two different weight bullets with fairly comparable BCs, but ideally, it would take two shooters of fairly equal ability, each shooting a different rifle/bullet, side-by-side, concurrently. One approach would be to center POI for both rifles on the targets at some specified distance under essentially calm conditions (i.e. no wind zeroes), then shoot them again at some point later in the day after the wind had picked up, with both shooters still holding dead center. It wouldn't take more than 10 or 20 shots each to form a couple nice groups with each load, the center-points of which could then be directly measured for horizontal/vertical deflection and compared.

The conundrum involved in the use of light, high BC bullets is not new. Years ago, I came up with the idea of shooting the Berger 90 VLD in a .223 bolt rifle in F-TR. A few years prior to that time, several individuals in Canada and the UK had had some success using that combination. They had posted some specifics of the setups they used and the whole idea intrigued and appealed to me. As a result, I ordered a .223 Rem rifle purpose-built to shoot the 90 VLDs. At the time this occurred, I was aware of only one other shooter that was routinely using the .223 Rem with 90s in F-TR here in the States. There may have been a few more that I didn't know about, but my point is that it wasn't a commonly-used setup at the time. There were more than a few rolled eyes among my F-Class shooting friends, but that did not dissuade me. To make a long story short, it didn't take long to make believers of them. On paper, the performance of a 90 VLD at ~2850 fps is predicted to be just a tick better than a typical 185 Juggernaut load, which was considered to be the "go-to" bullet for F-TR shooters using .308 Win rifles at the time. Note that the 200+ gr bullets did not yet enjoy widespread in .308 Win/F-TR use at that point. I have used that rifle in F-TR matches from 300 yd to 1000 yd in the time since, and not once have I ever observed the 90 VLDs behaving differently in terms of wind deflection than is predicted by the JBM ballistic calculator. Yet there are people that still claim the 90s "don't shoot right" at 1000 yd. IMO - the most likely reason they incorrectly believe that has to do with recent advances made in the 30 cal bullets commonly used in F-TR over the last few years.

As I mentioned, the 90 VLDs from a .223 Rem are predicted (on paper) to have ever so slightly less wind drift than a typical 185 Jug load. In my hands, my scores at 1000 yd reflected that directly over some period of time. I would always score just slightly better with the .223 than with my Juggernaut loads. However, there are now several very good 200+ gr 30 cal bullet designs that are commonly used in F-TR. Not surprisingly, the 90 VLDs give up quite a bit to loads with those bullets at 1000 yd. But so, too, do 185 Juggernauts. That's why you rarely, if ever, see the very top F-TR shooters using 185 Juggernauts at big matches. They'd simply be giving up too much as compared to the newer, high BC 200+ gr bullets. It's not that the 90 VLDs behave aberrantly, it's simply that they (and the 185 Juggernauts) have been surpassed by better 200+ gr bullets with higher BCs. Unless I knew with absolute certainty that the conditions would be relatively mild, I would not use one of my .223 F-TR rifles shooting 90s as a first choice at an important long-range match. In fairness, the 90s do have an additional issue in that it's very difficult to load the relatively small .223 Rem case to the same low ES/SD values that are readily achievable in .308 Win loads. So they give a tad more vertical at 1000 yd. However, in my hands that's usually a minor concern relative to the amount of wind deflection they are giving up as compared to the 200+ gr bullets in .308 Win. It's not that you can't shoot the 90s with almost sickening precision under fairly benign conditions, it's that your competitors shooting 200s will have a marked advantage at 1000 yd when the wind comes up. That is exactly the same reason that few top F-TR competitors are using 185 Juggernauts at 1000 yd in this day and age. Nonetheless, the value of the .223 with 90s has become much better appreciated in the last few years, especially for MR competitions, where you will often find a lot more of them on the firing line than there were several years ago.

However, the 131 gr .257 Blackjack bullet is a whole different animal, IMO. With an advertised G7 BC of 0.345, the predicted wind deflection for a Blackjack with ~2940 fps muzzle velocity at 1000 yd compares very favorably to that of a 180 Hybrid pushed at 2820 fps. The 180 Hybrid G7 box BC is ~0.349, its pointed G7 BC should be a little over 0.360). A 2940 fps velocity should be readily achievable with the 131 gr Blackjack using either the 6.5 Creedmoor or 6.5x47 parent cases.

It has always been my understanding that when loaded to equal pressure, the heavier, higher BC bullet will always show less wind deflection than the lighter, lower BC bullet, even though it will have a slower muzzle velocity. However, the Blackjack bullet is a different scenario. Here we have a lighter bullet with almost, but not quite equal BC to the 180 Hybrid. More importantly, the difference in BCs is close enough that the lighter bullet's slight BC deficit can be compensated for by its greater velocity. So on paper, wind deflection at 1000 yd should be identical, which brings us back to the question of whether the lighter bullet will behave aberrantly with respect to ballistic calculator predictions, solely because of its lesser mass. My experience and intuition suggests that in fact, this does not happen. I believe that bullets generally behave as their BC suggests they should, regardless of their mass, because the relative bullet mass is already taken into account in the BC. Nonetheless, I'm sure that others may have a different opinion, and keep their belief that the heavier bullet will generally win out.

Ultimately, it may be that only empirical side-by-side testing of rifles/loads with the two bullets will answer the question to everyone's satisfaction. I'd really love to see some side-by-side testing with the Blackjacks and 180 Hybrids. I've been watching this bullet since it was first advertised with the idea of of having my first F-Open rifle purpose-built to shoot it, much the same as I did with my first .223/90 VLDs. However, in my mind there is an even more critical question regarding this bullet than its apparent BC and wind behavior. Specifically, can they be loaded with the equal precision to the ~180 gr .284 bullets against which they would be competing? Not to take anything away from the Blackjack bullet or its manufacturers, but it is still somewhat of an unknown, at least to me, with regard to how easy it is to load and tune. There are at least two recent 30 cal bullet offerings with exceptionally high BCs for which many F-TR shooters have had extreme difficulty developing consistent loads/precision. At this point, it's very difficult to pinpoint the exact cause for this behavior, but the number of people that have experienced it suggests it's not an anomaly. In my mind, only time will tell how easy to load/tune the Blackjack bullet is with regard to an anticipated use in F-Class. shooting.

That’s more my concern with the blackjack. It’s a very long bullet for its caliber (which is why it’s got a high BC), but as we’ve seen with the super long Sierra .30s, that can be tough to keep in check. It may turn out that it’s better for PRS than F class.

The actual manufacturer is Sierra, so they should be of good quality. And I do think the .25 is an under appreciated caliber that may prove to be a good optimization for 1000 yard shooting. It’s just a matter of settling on a balanced bullet design. Blackjack started with a BC monster, but I know they’ve got plans for more. It will be interesting to see how it plays out.

 

[6fatrat]

Keith
I applaud you for staying on point.
I think your proposed experiment is a great idea, but of course the world is still flat.
Steve Bair

 

[Ballisticboy]

I see how the rifle can impact the drag characteristics, but if you assume equal muzzle velocity, what is the mechanism by which a shooter can influence drag? Minor variations in an initial lateral velocity due to differences in recoil control?

A shooter is like the recoil mechanism and the carriage on an artillery weapon. What the shooter is doing as the bullet nears or leaves the barrel will affect the initial yawing rate on the bullet as it leaves the gun just as mounting the same artillery gun on a different chasis will affect how a shell yaws on leaving the gun. Once you have an initial yawing rate you are dependent on the dynamic stability of the bullet to damp out the yaw and dynamic stability is not very good on some bullets particularly long low drag bullets. Different yaw rates will need different distances to damp down to small values giving different drag values at the start of the trajectory.

 

[dcali]

A shooter is like the recoil mechanism and the carriage on an artillery weapon. What the shooter is doing as the bullet nears or leaves the barrel will affect the initial yawing rate on the bullet as it leaves the gun just as mounting the same artillery gun on a different chasis will affect how a shell yaws on leaving the gun. Once you have an initial yawing rate you are dependent on the dynamic stability of the bullet to damp out the yaw and dynamic stability is not very good on some bullets particularly long low drag bullets. Different yaw rates will need different distances to damp down to small values giving different drag values at the start of the trajectory.

This makes perfectly good sense. Are you aware of any work that's been done that would quantify the impact of the shooter on drag (or for that matter aerodynamic jump)? I always just figured it was not terribly significant, but admittedly, that was just a (possibly poor) assumption.

 

[Ballisticboy]

This makes perfectly good sense. Are you aware of any work that's been done that would quantify the impact of the shooter on drag (or for that matter aerodynamic jump)? I always just figured it was not terribly significant, but admittedly, that was just a (possibly poor) assumption.

I am not aware of any specific trials on rifle shooters, it would be quite difficult to do. Most shooters refuse to stand in front of a radar (something to do with wanting families) which makes tracking the bullets to the degree of accuracy required quite difficult. There is however plenty of evidence in other calibres to support this supposition. We always used to insist on carrying out fire control model validation trials on weapons if anything had been done to the gun or its support. There were plenty of occasions when the same projectile would have different drag laws when fired from the same gun mounted on different plarforms. This is why range and accuracy trials should always be carried out with the gun and its mounting in its combat configuration. Hence since the shooter is the rifles "platform" it is logical that they to will affect the projectile drag particularly since the dynamic stability is usually pretty marginal.

 

[Keith Glasscock]

Keith, are you suggesting that the greater the surface area of the bullet the greater the impact impact on the bullet from the environment as it travels along the path?

I'm more suggesting that the initial acceleration of the bullet in a downwind direction could vary based on its mass and the forces applied to it by the still off-axis flow from a crosswind. While everything should be solid once in stabilized flight, there will be a period of time wherein the bullet is transitioning into stabilized flight (pointed directly into the relative wind).

 

[Keith Glasscock]

I don't believe that a test using two different rifles and two different shooter will result in true evaluation. Every barrel is different, more or less drag, sharper or smoother rifling, etc.
If we were to assume that you could find two barrels that will have almost neglegent differences you could set up two mounted test guns that are tied together. This would insure that you are aiming both the same. It would be easy enough to fire both at the same time and measure the difference at the target. But this would not tell you the strengths of the wind at different points along the path at the time the bullet travels through it.
The only way to do this would be to utilize a test chamber where the wind and its direction could be controlled and measured. To fire the rounds, you would have to use a single test gun that is mounted to remove the shooter from the equation and to ensure that different barrels would not have an effect on the outcome.

That perfectly explains the intent of the proposed test. The purpose would be to determine if mass creates any differences between two otherwise BC equal projectiles. Control measures would have to be put into place to ensure that the variations in BC do not affect the quality of the results.

The question comes down to what affects us as competitive shooters. In our case, we assume that we will make a wind-call error. Differences in distance from center as a result of that error is the only thing of interest.

I'd suggest that all rounds fired would have to have both initial and terminal velocities measured. if muzzle velocities and impact velocities are equal, we have defined identical BC by nature of the method used to measure it to begin with.

This should not be done at a short range. I'd suggest that it be done at 1000 yards to maximize the potential for a "readable" set of data.

If 20 to 40 rounds are fired per rifle over a period of hours when the wind is changing (a full direction change would be ideal), the total width of the resultant groups should tell us whether the mass of the bullet makes any significant difference.

 

[Papa Charlie]

I would very much like to see the results of such a test. Again I believe for the results to be of any value it would have to be done in a controlled environment where we can be assured as much as humanly possible that each round fired is reacting to the same exact influences.

 

[Ballisticboy]

I'm more suggesting that the initial acceleration of the bullet in a downwind direction could vary based on its mass and the forces applied to it by the still off-axis flow from a crosswind. While everything should be solid once in stabilized flight, there will be a period of time wherein the bullet is transitioning into stabilized flight (pointed directly into the relative wind).

The time for a bullet to turn into the airflow is minute so any effect will also be minute. However, the bullet will not simply turn and point, there will be yawing about the zero point. The detailed stability characteristics of each bullet design will affect the yawing of the bullet.

I'd suggest that all rounds fired would have to have both initial and terminal velocities measured. if muzzle velocities and impact velocities are equal, we have defined identical BC by nature of the method used to measure it to begin with.

This would not ensure that the shape of the different bullet drag curves and hence their wind response is identical and is one of the fundamental flaws in the BC method which assumes all drag curve shapes are identical. The bullets would have to be tracked by radar and display identical drag curve shapes, identical BCs between two points or a number of points would be insufficient.
Rather than try to find any such bullets, to say nothing of the cost of using a suitable doppler tracking radar (a fixed head radar would not be sufficient), initial studies would be better using six degree of freedom modelling. Suitable data can be generated using identical drag curve shapes and bullet characteristics. The modelling would tell you if there is any significant effect and could also direct any experimental work with a full knowledge of the significant variables.
In that way wasted effort could be avoided and the results would have more scientific significance.

 

[davidjoe]

Ned, totally agree that that the 90 grain .223’s actual drift belies predictions. Something does seem to be missing. I have wondered for a long time, (without reading up on answers that probably do exist), if the deformations (grooves) caused by rifling are fully accounted for. Surely those indentations increase drag. They are about the same depth regardless of caliber. They are relatively longer on longish, smaller caliber bullets. Longer bullets also have to spin faster, increasing any effect from groove drag. Therefore it seems reasonable that grooves have a greater effect on smaller caliber bullets. That’s probably baked-in already to some published BC’s if based on observation, but perhaps not for calculations based solely on the bullet’s physical characteristics as they stand when shipped.

 

[TC260]

That perfectly explains the intent of the proposed test. The purpose would be to determine if mass creates any differences between two otherwise BC equal projectiles. Control measures would have to be put into place to ensure that the variations in BC do not affect the quality of the results

I don't have the ballistics knowledge to answer your question, I'm just trying to understand your question...

It sounds like you may be asking a question of momentum? Possibly? In other words, if two bullets of different masses, fired at the same time, with the same BC, are being blown in the -Vy direction, they each have momentum in that direction. If they encounter a wind shift from the +Vy direction, their momentum in the -Vy direction has to be overcome before they shift to moving in the +Vy direction.

If those two projectiles fired at the same time encounter multiple wind switches on the way to the target, are both projectiles going to shift direction the exact same distance both -Vy and +Vy regardless of their differences in mass (momentum) because the BC's are the same? Something similar to that anyway?

Again, I don't have the answers, just trying to clarify the question

 

[243Mendoza]

I don't have the ballistics knowledge to answer your question, I'm just trying to understand your question...

It sounds like you may be asking a question of momentum? Possibly? In other words, if two bullets of different masses, fired at the same time, with the same BC, are being blown in the -Vy direction, they each have momentum in that direction. If they encounter a wind shift from the +Vy direction, their momentum in the -Vy direction has to be overcome before they shift to moving in the +Vy direction.

If those two projectiles fired at the same time encounter multiple wind switches on the way to the target, are both projectiles going to shift direction the exact same distance both -Vy and +Vy regardless of their differences in mass (momentum) because the BC's are the same? Something similar to that anyway?

Again, I don't have the answers, just trying to clarify the question

a=F/m and BC is a measure of F/m so according to this equation, both bullets should react the same to wind shifts.

 

[dcali]

Even if you got the rest of the test perfect, all you'd be measuring is the difference in two bullets' drag characteristics, neither of which are perfectly described by a BC. Getting two bullets with exactly the same drag curve but different weights and identical BCs is not a simple thing.

Once you drop the concept of BCs and start looking at actual drag functions, the idea that mass matters in addition to BC is nonsensical because ballistics calculators do not use BC in the math at all - they use drag coefficients and mass. BCs do not exist outside of theory - they're just a clever way to input a mass and a drag curve all wrapped in one number. The calculator reverses this mathematical trick under the hood.

I guarantee you that regardless of how mass is input into the calculator, it is fully accounted for.

 

[Ned Ludd]

Even if you got the rest of the test perfect, all you'd be measuring is the difference in two bullets' drag characteristics, neither of which are perfectly described by a BC. Getting two bullets with exactly the same drag curve but different weights and identical BCs is not a simple thing.

Once you drop the concept of BCs and start looking at actual drag functions, the idea that mass matters in addition to BC is nonsensical because ballistics calculators do not use BC in the math at all - they use drag coefficients and mass. BCs do not exist outside of theory - they're just a clever way to input a mass and a drag curve all wrapped in one number. The calculator reverses this mathematical trick under the hood.

I guarantee you that regardless of how mass is input into the calculator, it is fully accounted for.

This is very true. More often than not, the next higher weight class of bullets will also have noticeably higher BCs. In most cases, when two bullets of dissimilar weight and BC are loaded to equal pressure, the heavier, higher BC bullet will exhibit less wind deflection, even though it will have a markedly lower muzzle velocity. In other words, increased velocity will generally not make up for a substantial BC deficit. However, there are a few cases where a lighter bullet with an exceptionally high BC for its weight can be pushed just fast enough at reasonable operating pressures to overcome the BC deficit as compared to the next higher weight class of bullets - one example being the 168 Hybrid versus 185 Juggernaut.

Although rigorously controlled experiments are always desirable, they are likely not necessary for the sole purpose of determining whether a lighter bullet with a very high BC for its weight displays anomalous wind deflection behavior. A simple direct comparison, even with a few caveats, should reveal whether there is a substantial difference.

Davidjoe - perhaps I didn't state my experience clearly enough. In my hands, I have never noticed the 90 VLDs to experience markedly greater than expected wind deflection. There are probably not many shooters that have fired more 90 VLDs in F-TR matches than I have. In my hands, they have always behaved exactly as would be expected based on their BC and muzzle, at all ranges out to 1000 yd. The point I was trying to make was that even though the 90 VLD is a very good bullet design and behaves as predicted by ballistic calculators, it is at a noticeable wind disadvantage at 1000 yd against higher BC 200+ gr 30 cal bullets. For that reason, someone shooting a .223 Rem with 90s in a 1000 yd F-TR match on a windy day against other equally skilled shooters using 200s in a .308 may not fare so well. However, that is a very different thing than suggesting the 90 VLD exhibits much greater than expected windage at 1000 yd.

 

[284winner]

Interesting opinion.

I appreciate your input.

The question was whether anyone has or is doing real-world testing to validate the BC to wind drift relationship?

Yes. ELR competitions prove that heavier bullet/caliber combinations are superior in wind at the longest ranges. If the theory that the same BC bullets regardless of weight are equal at ELR, why don't we see the 28 Nosler shooting the 180 elds (.8) or 195 Berger's( .81) both on par with the 338 LM only much faster ? Still, they are not wind worthy at the ELR. Fact is hard to compare to theory.

 

[Keith Glasscock]

Even if you got the rest of the test perfect, all you'd be measuring is the difference in two bullets' drag characteristics, neither of which are perfectly described by a BC. Getting two bullets with exactly the same drag curve but different weights and identical BCs is not a simple thing.

Once you drop the concept of BCs and start looking at actual drag functions, the idea that mass matters in addition to BC is nonsensical because ballistics calculators do not use BC in the math at all - they use drag coefficients and mass. BCs do not exist outside of theory - they're just a clever way to input a mass and a drag curve all wrapped in one number. The calculator reverses this mathematical trick under the hood.

I guarantee you that regardless of how mass is input into the calculator, it is fully accounted for.

What a perfectly circular argument.

I didn't come here to argue. Instead, I came looking to find out if anyone has done the testing.

I suspect that more testing has been done than is immediately apparent. I'll do some inquiry elsewhere.

 

[davidjoe]

Yes. ELR competitions prove that heavier bullet/caliber combinations are superior in wind at the longest ranges. If the theory that the same BC bullets regardless of weight are equal at ELR, why don't we see the 28 Nosler shooting the 180 elds (.8) or 195 Berger's( .81) both on par with the 338 LM only much faster ? Still, they are not wind worthy at the ELR. Fact is hard to compare to theory.

Some might claim that hits and misses are difficult to spot with smaller bullets. But presently it’s becoming harder and harder to campaign an accurate .50 in ELR under the mechanism of declining weight restrictions. I’ve suspected that others long-since arrived at that exact performance conclusion you state, and want to encourage ELR competition that’s not akin to a strong man trying to ring a bell with a sledge hammer at the county fair. Your original point also captures that question of why the .223 isn’t more popular at 1,000. By the charts it should have shot inside .308’s until the most recent .308 bullets came out.