Wind drift - theory vs reality

[284winner]

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.

Yes true. I believe the 223 DOES rival the 308 with equal velocities and BCs however beyond a certain range, I believe the heavier bullet will rival the lighter bullet in wind with more predictable shot placement. Correct, bullet impact is very tough to glass with a 338 at 2000 yards let alone a .284 bullet, but again, if the 284 bullet can make it effectively on target as often as the larger, heavier bullets of the 338, there would be alot of 7mm rifles in those competitions. I have not seen a 7mm rifle in any of those events. The 30 heavies are becoming more of an interest to some of the ELR guys from what I've been reading here on this forum. Will be interesting as bullets get better and better, what cartridges will show up in those events.

 

[davidjoe]

“Do we really know how BC and wind drift interact?”

I say no, not by BC alone. BC only relates a subject bullet to the traits/flight traits of one of several reference projectiles of different shapes, that weigh one pound, apparently through still air.

If no comparison to the reference bullet’s wind drift need be made in determining BC, then resistance to drift is not captured information. Some subtle variations of subject bullet shape might resist lateral wind better than others. Mass though, I think would already get accounted for in velocity retention relative to the reference projectile.

 

[BronzeArcher]

I think the simultaneous firing test described by the original poster is an excellent idea and about the best that can be done to support or refute the theory that equal BCs (or drag curves of one prefers) and muzzle velocities will yield the same wind drift (independent of mass and independent of side profile.)

Since all theories are ultimately supported or disproven by experiments, I tend to be skeptical of arguments that amount to one theory (modified point mass wind drift predictions) being supported by another theory (6DOF wind drift predictions.) Assertions of this nature demonstrate a fundamental ignorance of how science works.

Theory A is supported by theory B is never as compelling an argument as "here's the experimental data supporting theory A."

 

[dcali]

I think the simultaneous firing test described by the original poster is an excellent idea and about the best that can be done to support or refute the theory that equal BCs (or drag curves of one prefers) and muzzle velocities will yield the same wind drift (independent of mass and independent of side profile.)

Since all theories are ultimately supported or disproven by experiments, I tend to be skeptical of arguments that amount to one theory (modified point mass wind drift predictions) being supported by another theory (6DOF wind drift predictions.) Assertions of this nature demonstrate a fundamental ignorance of how science works.

Theory A is supported by theory B is never as compelling an argument as "here's the experimental data supporting theory A."

I agree in general, but in this case theories A and B are supported by massive amounts of testing with equipment and budgets the shooting industry can only dream about. And they agree with each other within the margin that would be expected given their differences.

Edit:
The theory (it's really just one - point mass is just a special case of 6DOF) is still just Newton's second law fed with empirical data. The big picture here is that if you want to rely on scientific tests, they've already been done extensively and with far more precision that we are capable of as shooters - that's where all the drag data comes from. We don't need to retest that. If you have doubts about the theory, take it up with Newton.

 

[Ballisticboy]

I think the simultaneous firing test described by the original poster is an excellent idea and about the best that can be done to support or refute the theory that equal BCs (or drag curves of one prefers) and muzzle velocities will yield the same wind drift (independent of mass and independent of side profile.)

Since all theories are ultimately supported or disproven by experiments, I tend to be skeptical of arguments that amount to one theory (modified point mass wind drift predictions) being supported by another theory (6DOF wind drift predictions.) Assertions of this nature demonstrate a fundamental ignorance of how science works.

Theory A is supported by theory B is never as compelling an argument as "here's the experimental data supporting theory A."

A couple of points. first as I said before an equal BC does not equate to an equal drag curve. This is why even the small arms industry is now moving away from BCs as shooters, particularly ELR shooters, must try to do if they are concerned with the minutiae of shooting.
Second, at no point has anyone suggested that theory B is used to prove theory A. What I was suggesting was that proven ballistic models, as used by government ballistic labs all over the world for basic research, not theories, should be used first to establish key variables. It is basic fundamental science to carry out all such research first before any experiments. Carrying out any experiment without first establishing and controlling all the variables in order to isolate exactly what it is you are trying to test will merely produce unusable data and lead to misinterpretation of the results. Experimental data supporting theory A is only as good as the scientific integrity of the experimental method used.
To the OP I would say by all means carry out your side by side test but, first recognise the limitations of the method used, and second make sure you are controlling all your variables and that you have a full data set necessary to isolate and analise the results on the variable in which you are interested. Failure to do so will only result in unsupportable or misleading conclusions. As someone who spent many years carrying out all types of external ballistic tests for various militaries and research establishments I know it is only too easy to miss something which completely wrecks your test results.
So do you feel lucky?

 

[davidjoe]

I think the simultaneous firing test described by the original poster is an excellent idea and about the best that can be done to support or refute the theory that equal BCs (or drag curves of one prefers) and muzzle velocities will yield the same wind drift (independent of mass and independent of side profile.)

Since all theories are ultimately supported or disproven by experiments, I tend to be skeptical of arguments that amount to one theory (modified point mass wind drift predictions) being supported by another theory (6DOF wind drift predictions.) Assertions of this nature demonstrate a fundamental ignorance of how science works.

Theory A is supported by theory B is never as compelling an argument as "here's the experimental data supporting theory A."

Extreme illustration: I just visualize two different shaped projectiles that resist pure headwind in a wind tunnel equally well and that yield the same BC. Imagine that one of them is a fin-stabilized sabot while the other is a finless boattail design. If the angle of the wind was slightly altered for both, so that the fins become exponentially more “visible” to the wind, is not the finned object that resists head wind equally well, not at a severe disadvantage against lateral deflection.

 

[expiper]

you guys are way over my head,,,back in the 70's when many shooters used .308 with 168 +/- gr bullets in their rail guns ,,,during evening practice sessions we would sit (2-6 shooters) on concetuive benches six-eight foot spacing and fire simultaneously snd compare bullet deflection in various wind conditions,,,,the little 6mm's using 62-68 gr bullets would always have less drift ,,,,althouh the BC was less ,,the velocity was higher and made the difference,,,this was done at 100-200 yds,,,,,Roger

 

[dcali]

OK - interesting side story. A lot of you who know me know that I'm very much in the "believe the math" camp of this sort of thing, and that I feel like field reports from shooters are unreliable and often just mistaken. But I read about an interesting case where those field reports were pretty much spot on.

In the early 70s Engineers were getting reports that 105mm artillery shells were drifting to the left, when every bit of science out there says they must drift to the right (because of the right twist). At some point the BRL decided to look into it and discovered that it was due to shooting in higher angle trajectories than normally possible by essentially putting the artillery piece on a hill (sort of like using a sight base with built in elevation). What was happening is that the arc was so steep that the round was not nosing over at the top, and was falling down base-first, effectively reversing the spin direction and causing the leftward drift.

So crazy field reports aren't always wrong. In this case they are, though

 

[BronzeArcher]

I agree in general, but in this case theories A and B are supported by massive amounts of testing with equipment and budgets the shooting industry can only dream about. And they agree with each other within the margin that would be expected given their differences.

Edit:
The theory (it's really just one - point mass is just a special case of 6DOF) is still just Newton's second law fed with empirical data. The big picture here is that if you want to rely on scientific tests, they've already been done extensively and with far more precision that we are capable of as shooters - that's where all the drag data comes from. We don't need to retest that. If you have doubts about the theory, take it up with Newton.

Sort of, but there are a number of untested assumptions in McCoy's 6DOF model, and accurately determining all the needed input coefficients is challenging and requires spark range facilities (or the equivalent). The needed input coefficients have only been accurately determined for a very small percentage of small arms projectiles available today. The 168 SMK is one of very few match bullets to have been tested, for example. (At least that is known to have been tested in the open literature.)

So how well McCoy's 6DOF model really represents Newton has left room for doubt. The question is not really whether F=ma, but rather the lateral wind drift force is only a function of the frontal drag. This is the essential point of physics in question. There are some other open questions about the aerodynamic forces, but this is the big one for the wind drift question.

But as far as experimental validation of McCoy's 6DOF goes, I'd be more convinced if you could cite the published data validating wind drift predictions with an accuracy better than 10% or so for spin stabilized projectiles of varying masses and side profiles. I am doubtful this data exists in the public domain, not only because I've looked and come up empty. I've also asked around quite a bit over the years and no one can point to it. Finally, I realize how challenging the experimental validation of McCoy's wind drift prediction would be in the first place - one needs an accurate measurement of the wind field all along the projectile path, and that's very hard to do.

That's the problem Keith's simultaneous firing experiment addresses, at least in part. One need not know the wind field at every point in the trajectory if one simultaneously launches two different projectiles with the same drag curve and initial velocity. If the two projectiles are close enough in space, then at least one knows that they are approximately the same wind field - or at least closer to the same wind field than they could ever really be measured in an experiment purporting to validate McCoy's predictions directly. McCoy's theory predicts the same wind drift (or very nearly the same) for both projectiles.

If it turns out that one kind of projectile (heavier, for example) reliably exhibits less wind drift than the other, then the McCoy's wind drift predictions have been falsified. I'm not talking one data point - consider an experiment in which 100s of rounds are fired simultaneously.

 

[dcali]

Sort of, but there are a number of untested assumptions in McCoy's 6DOF model, and accurately determining all the needed input coefficients is challenging and requires spark range facilities (or the equivalent). The needed input coefficients have only been accurately determined for a very small percentage of small arms projectiles available today. The 168 SMK is one of very few match bullets to have been tested, for example. (At least that is known to have been tested in the open literature.)

So how well McCoy's 6DOF model really represents Newton has left room for doubt. The question is not really whether F=ma, but rather the lateral wind drift force is only a function of the frontal drag. This is the essential point of physics in question. There are some other open questions about the aerodynamic forces, but this is the big one for the wind drift question.

But as far as experimental validation of McCoy's 6DOF goes, I'd be more convinced if you could cite the published data validating wind drift predictions with an accuracy better than 10% or so for spin stabilized projectiles of varying masses and side profiles. I am doubtful this data exists in the public domain, not only because I've looked and come up empty. I've also asked around quite a bit over the years and no one can point to it. Finally, I realize how challenging the experimental validation of McCoy's wind drift prediction would be in the first place - one needs an accurate measurement of the wind field all along the projectile path, and that's very hard to do.

That's the problem Keith's simultaneous firing experiment addresses, at least in part. One need not know the wind field at every point in the trajectory if one simultaneously launches two different projectiles with the same drag curve and initial velocity. If the two projectiles are close enough in space, then at least one knows that they are approximately the same wind field - or at least closer to the same wind field than they could ever really be measured in an experiment purporting to validate McCoy's predictions directly. McCoy's theory predicts the same wind drift (or very nearly the same) for both projectiles.

If it turns out that one kind of projectile (heavier, for example) reliably exhibits less wind drift than the other, then the McCoy's wind drift predictions have been falsified. I'm not talking one data point - consider an experiment in which 100s of rounds are fired simultaneously.

It’s not really McCoy’s work - there are decades of research and experiments by many people that developed modern ballistics. The scale and scope of this stuff far outstrips what one man can accomplish. Much of the data that people want is out there. It’s just not in the form people seem to want.

But what you’re talking about is simply the uncertainty of inputs, not the fundamental idea that mass is mass and you can’t double count it and have your model make sense. If a proposed model isnt compatible with classical physics, it’s wrong and there isn’t any point in testing it. We can’t just say F = 1.1*MA just because shooters say so. I don’t need a test to say that’s wrong.

The 6DOF model is an application of classical physics. Any errors come from factors that have been ignored (there are a few small things that are purposefully neglected) or from uncertainty in the inputs. Is there room to sharpen the detail for competition bullets? Absolutely. But that is a far different question than “do heavy bullets outperform light bullets with the same BC and muzzle velocity.”

 

[davidjoe]

“The question is not really whether F=ma, but rather the lateral wind drift force is only afunction of the frontal drag.”

If it was only a function of frontal drag, interstate semi trucks with and without trailers caught in gusts would drive the same.

 

[Ballisticboy]

OK - interesting side story. A lot of you who know me know that I'm very much in the "believe the math" camp of this sort of thing, and that I feel like field reports from shooters are unreliable and often just mistaken. But I read about an interesting case where those field reports were pretty much spot on.

In the early 70s Engineers were getting reports that 105mm artillery shells were drifting to the left, when every bit of science out there says they must drift to the right (because of the right twist). At some point the BRL decided to look into it and discovered that it was due to shooting in higher angle trajectories than normally possible by essentially putting the artillery piece on a hill (sort of like using a sight base with built in elevation). What was happening is that the arc was so steep that the round was not nosing over at the top, and was falling down base-first, effectively reversing the spin direction and causing the leftward drift.

So crazy field reports aren't always wrong. In this case they are, though

Reminds me of a case which I got to be involved in. Some proof shot were fired at an angle of about 60 degrees when firing them above about 45 degrees was banned. The two shots fired landed 4km to the left of the line of fire. The rounds did not turn at all at the top of the trajectory and became magnus rotors as they came back down sideways into a farmers field. He was not happy.

 

[243Mendoza]

“The question is not really whether F=ma, but rather the lateral wind drift force is only afunction of the frontal drag.”

If it was only a function of frontal drag, interstate semi trucks with and without trailers caught in gusts would drive the same.

the side wind of bullet is a very small fraction of the head wind compared to a truck. Besides, the bullet will veer into the wind, so it faces the wind.

 

[davidjoe]

the side wind of bullet is a very small fraction of the head wind compared to a truck. Besides, the bullet will veer into the wind, so it faces the wind.

Totally agree it’s a tiny fraction. But any value more than zero might help explain why identical BC bullets drift slightly differently. Also agree that it will want to weathervane into wind. Problem there is that its momentum toward aimpont dictates, so weathervanibg turns it slightly broadside and this is where the shape other than its “front” would matter.

 

[243Mendoza]

Totally agree it’s a tiny fraction. But any value more than zero might help explain why identical BC bullets drift slightly differently. Also agree that it will want to weathervane into wind. Problem there is that its momentum toward aimpont turns it slightly broadside and this is where the shape other than its “front” would matter.

True, this is why Cd is usually a function of angle of attack and takes the usual shape of a parabola, so Cd is a minimum at zero angle of attack and at very small angles.

 

[dskogman]

The use of E targets can actually provide some very interesting real world data relative to these discussions. I have been collecting all the individual shot data in our club matches and correlating them vs time/classification/ discipline/etc. It’s still early but the shot dispersion charts do not look particularly like the theoretical ones I see posted no matter how it’s sliced. I’m pretty sure it’s due to real world variations in wind direction/velocity over the length of the range and terrain induced turbulence which we have a lot of.
I have data for three 1000 yd matches so far with 30-40 shooters each for a total of over 5k record shots and the shot dispersion is almost perfectly circular. There will be brief periods when shots drift left or right or up or down or combinations of two but in the aggregate is pretty uniformly distributed radially.
I’ve thought for some time that vertical effects are under accounted at least at our range and this seems to bear that out. I’d like to see how this compares to other ranges.

 

[Ballisticboy]

Also agree that it will want to weathervane into wind. Problem there is that its momentum toward aimpont dictates, so weathervanibg turns it slightly broadside and this is where the shape other than its “front” would matter.

When the bullet turns in a cross wind it turns to face the total airflow vector so that although relative to its forward movement it is moving slightly broardside it is pointing directly into the relative airflow thus there is no effect of the shape of the side of the bullet.
There still seems to be some confusion with drag and BCs. Just because two bullets have the same BC it does not mean they have the same form of drag curve. In most cases with the method used to calculate BC it just means they have the same start and end velocities over a given range. It does not even mean they have the same time of flight over a given range or the same velocity profile. Different shaped bullets have different shaped drag curves which will give different velocity profiles. When the different velocity profiles are combined with the wind profile then it is entirely possible you may get small differences in down wind drift. It is not a failure of the mathematics or the theory used but a failure of the methods in which they are being applied.

 

[dcali]

Another wrinkle is that BC is somewhat subjective. There are lots of ways to look at a drag curve and assign a BC to it. The real curve and the standard curve will only match so well. How you decide to lay them on top of each other will determine the BC. For our bullets and a G7 curve we’re most interested in the range of ~Mach 1.2 to Mach 2.5, so you would hope that BCs are chosen to fit well in that region, but that’s not the only way to do it. It requires some judgement.

 

[davidjoe]

When the bullet turns in a cross wind it turns to face the total airflow vector so that although relative to its forward movement it is moving slightly broardside it is pointing directly into the relative airflow thus there is no effect of the shape of the side of the bullet.
There still seems to be some confusion with drag and BCs. Just because two bullets have the same BC it does not mean they have the same form of drag curve. In most cases with the method used to calculate BC it just means they have the same start and end velocities over a given range. It does not even mean they have the same time of flight over a given range or the same velocity profile. Different shaped bullets have different shaped drag curves which will give different velocity profiles. When the different velocity profiles are combined with the wind profile then it is entirely possible you may get small differences in down wind drift. It is not a failure of the mathematics or the theory used but a failure of the methods in which they are being applied.

That sounds pretty logical, after all if you mounted a weathervane to a moving car you could never stress it by changing directions so long as it could freely spin, which bullets can do. Very unlike a ship steering aganst a current or a plane (or the trailered truck) which can present their profiles to force.

Yes, there’s definitely a wide-held assumption that BC correlates precisely to wind drift for similar starting velocities. It’s all we have to work with, and seems to be what the trajectory calculators rely on. It only correlates “closely”, it seems. But it’s all the information the manufacturers give us on which to base close call decisions, such as picking between a fast 6 and a slower 7.

 

[davidjoe]

Ballistic,... I think on reflection a bullet’s response to crosswind is merely a mix of, and between, a windvane’s supreme efficiency in equalizing pressure on both of its sides, and say, a truck. There is slippage especially in transient conditions and I also think a good deal of variance between different bullets.