Spin Drift

damoncali

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Mar 19, 2011
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It's not a math problem so much as a data problem. Aerodynamic coefficients are simply difficult to nail down with the precision that a lot of ELR shooters want. We know how to calculate drift. Hell - decades ago, they figured out how to calculate the drift when a howitzer projectile is shot 80 degrees into the air, fails to nose over, and falls backwards, reversing the drift on the way down.

It's the uncertainty in that calculation that's being pushed. We can calculate the moments of inertia to near perfection, but the overturning and lift coefficients are still kind of spotty, especially for new ELR bullets. Measuring or calculating those with more certainty is what is needed, not a new way to do the math. The BRL and others figured this out long ago. They just didn't care about this level of detail.
 
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Jim Boatright

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Feb 21, 2018
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Just thought I would mention another data point. I ran Bryan's AB point-mass solver for one of my new bullets to be fired at Whittington NM later this year. I used the McDRAG calculated BC(G7) multiplied by 1.124 to agree with David Tubb's 1000-yard measurements of similar design hyper-stable 338-caliber copper bullets earlier this year. The subject bullet is the 375-caliber, 343.6-gr Mk II bullet to be prototyped by Dan Warner in a few weeks. Other good copper solids should work similarly. The G7 BC I used was 0.4954. Whittington is at a 6500 feet elevation above sea-level, so local absolute air pressure should be 23.54 inHg according to ICAO. For an ambient air temperature of 65 degrees F, the speed of sound should be about 1123 fps. Based on this bullet ogive shape and dimensions, transonic buffeting should not begin until this bullet slows to Mach 1.1 or 1235 fps, and it should become completely subsonic at Mach 0.90 (1011 fps). I threw in 10-percent Relative Humidity as well.

I ran trajectories out to 4500 yards for various muzzle speeds from a 7.0-inch twist .375/.366 barrel for initial hyper-stability with initial Sg = 3.25. Basically, I found that the maximum supersonic range in yards approximately matches muzzle speed in fps between 3000 and 4500 fps. That is, if you want 3520 yards (2 statute miles) of supersonic flight, you need a launch speed of 3520 fps, which is feasible. Transonic flight requires just the next 200 yards of bullet travel before subsonic flight commences. You would need 28.7 mils of total muzzle elevation to zero at 3520 yards. The bullet still carries 1164 ft-lbs of kinetic energy at 1235 fps. Total drop from the bore axis is about 308 feet, so I am guessing about 6.5 feet of spin-drift at 2 miles (I did not run those numbers yet). Bryan estimates spin-drift at 11.3 feet and the Coriolis effect at 2.5 feet, also rightward.

EDIT: I ran the numbers for this Whittington atmosphere, and spin-drift at 1000 yards is just 2.2 inches. It is 4.0 feet at 3520 yards. The Scale Factor is 1.32 percent of drop. It actually required a MV of 3540 fps to reach 3520 yards at Mach 1.10 (1235 fps).
 
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Skookum

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May 6, 2017
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Do you still expect the nose to track with the trajectory at this rate of twist? Using the groove diameter this equals 18 calibers per turn. If so, at what rate of spin would you expect it to maintain a nose up orientation on the downrange side of max ordinate?
 

Jim Boatright

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Feb 21, 2018
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Do you still expect the nose to track with the trajectory at this rate of twist? Using the groove diameter this equals 18 calibers per turn. If so, at what rate of spin would you expect it to maintain a nose up orientation on the downrange side of max ordinate?
Yes. All rifle bullets track up to 0.1 degrees high at reasonable ranges. The tracking angle increases with range and time of flight in flat firing. If you are asking if you can detect it, no, not very likely. At somewhere around 70 degrees of muzzle elevation you might have the problem you seem to fear. Max range should be at about 30 to 35 degrees of muzzle elevation. Don't shoot straight upwards, or your bullets will come down backwards.
 
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damoncali

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Mar 19, 2011
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Interesting reading on high angle fire and it's impact on drift (because of base forward flight on the way down): https://catalog.hathitrust.org/Record/101827385 . It's about artillery, but the concepts should be similar for rifles. (Spoiler: you don't have to worry about it.) There is an angle at which the upwards drift is totally cancelled out by downward drift, which is cool in an academic sort of way.
 

Skookum

Knuckle Dragger
May 6, 2017
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Interesting reading on high angle fire and it's impact on drift (because of base forward flight on the way down): https://catalog.hathitrust.org/Record/101827385 . It's about artillery, but the concepts should be similar for rifles. (Spoiler: you don't have to worry about it.) There is an angle at which the upwards drift is totally cancelled out by downward drift, which is cool in an academic sort of way.
I wasn't speaking of high angle fire, though the effect you spoke of seems interesting.

I asked about the nose tracking less with the mean trajectory on the down range side of max ordinate, to ask this:
I wonder if it has been shown that a flat fired, hyperstabilized bullet shows some smaller fraction of spin drift from muzzle to max ord, and an increased fraction on the down range side of max ord due to the nose tracking at a different angle in relation to the mean trajectory.
 

Jim Boatright

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Feb 21, 2018
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I wasn't speaking of high angle fire, though the effect you spoke of seems interesting.

I asked about the nose tracking less with the mean trajectory on the down range side of max ordinate, to ask this:
I wonder if it has been shown that a flat fired, hyperstabilized bullet shows some smaller fraction of spin drift from muzzle to max ord, and an increased fraction on the down range side of max ord due to the nose tracking at a different angle in relation to the mean trajectory.
Cranz has quite a discussion of spin-drift in high-angle firing. He says the sane things.

Those high-angle effects are caused by 1) the projectile nearly coming to a halt at apogee, 2) the thinner air at flight apogee, and 3) the projectile re-accelerating as it falls. Have you ever had your rifle bullets speed-up during flight? Neither have I. This is all covered by the restriction of "flat firing," which is all we are interested in here. Show me a target with rifle bullets impacting "nose high," and then I will worry about "over-stabilizing" the bullets. Ain't gonna happen, though.
 
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TripleBull

This one goes to 11
Feb 13, 2017
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Those high-angle effects are caused by 1) the projectile nearly coming to a halt at apogee
But that only happens at very high angle. I thought we were talking ~35 degrees earlier - at that angle the forward velocity does not approach zero until ~impact.

Have you ever had your rifle bullets speed-up during flight?
My bullets, no, but I witnessed an ~vertical launch impact a nearby barn roof (and got the hell away from the lunatic that shot it).
 
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damoncali

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Mar 19, 2011
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I wasn't speaking of high angle fire, though the effect you spoke of seems interesting.

I asked about the nose tracking less with the mean trajectory on the down range side of max ordinate, to ask this:
I wonder if it has been shown that a flat fired, hyperstabilized bullet shows some smaller fraction of spin drift from muzzle to max ord, and an increased fraction on the down range side of max ord due to the nose tracking at a different angle in relation to the mean trajectory.
Are you talking about an increasing yaw of repose? Seems like yaw has been pretty well documented with 6DOF models.
 

Jim Boatright

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Feb 21, 2018
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Are you talking about an increasing yaw of repose? Seems like yaw has been pretty well documented with 6DOF models.
Not at all, DC. In the Coning Theory and Spin-Drift papers, I explained how and why the yaw-of-repose and spin-drift occur and numerically calculated SD to within 0.021-inch (Std Dev) of the PRODAS data for each millisecond over 1.693 seconds of a 1000-yard simulated flight. I was merely trying to explain that the coning bullet tracks the mean trajectory tangent direction (+V direction) with very small dynamic horizontal and vertical tracking error angles, always much less than 0.10-degree in flat firing. The tracking time constant for coning motion is one half the period of a coning cycle at 35 to 70 hertz. The yaw-of-repose attitude angle leads the rightward curving spin-drift and causes it. The downward arcing of the trajectory due to gravity leads the coning axis attitude downward and causes that nosing downward of the coning bullet. Neither attitude changes much in 7.5 to 15 milliseconds of flight time.
 
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Skookum

Knuckle Dragger
May 6, 2017
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Random report.

I have used the 2% of drop figure for spin drift on 4 different 6.5's, 2 different 300 winmags, 2 different 308's and one 338 Allen express. All different loads and distances from 500 yds to 1763 yds. That shit is balls on. Whatever difference there may be from the actual number is too small to see and too small to dial. This is my new rule of thumb.
 
Feb 7, 2013
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2% or 4% I get it. But if one can call wind on a "normal" Western State day even a tiny bit close to these #s --they are a hero of heros. Honestly, I let it and CE go and just deal with the optical disturbances (way larger than SD or CE) but not as sexy to talk about.. and work the damn wind the best I can..

I have see some of the best shooters in the world focus on SD/CE, make assume wind calls only to use a fucked spotting process and call misses (not fast corrections) completely ignoring scope units on a spotter without a reticle and taking feet and inches or parts of gongs..
 
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Skookum

Knuckle Dragger
May 6, 2017
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2% or 4% I get it. But if one can call wind on a "normal" Western State day even a tiny bit close to these #s --they are a hero of heros. Honestly, I let it and CE go and just deal with the optical disturbances (way larger than SD or CE) but not as sexy to talk about.. and work the damn wind the best I can..

I have see some of the best shooters in the world focus on SD/CE, make assume wind calls only to use a fucked spotting process and call misses (not fast corrections) completely ignoring scope units on a spotter without a reticle and taking feet and inches or parts of gongs..
We've already gone over these points. I have no desire to rebut them.

I have no idea how well others call wind, or what methods they prefer. I will tell you that I live in the Cascades just north of Seattle. I shoot in the mountains, I call wind in the mountains. I call for rifles from 223 all the way up to the 338's...and I do it well enough to see the difference.

Take it for what it is, one more piece of anecdotal evidence.
 
Feb 7, 2013
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We've already gone over these points. I have no desire to rebut them.

I have no idea how well others call wind, or what methods they prefer. I will tell you that I live in the Cascades just north of Seattle. I shoot in the mountains, I call wind in the mountains. I call for rifles from 223 all the way up to the 338's...and I do it well enough to see the difference.

Take it for what it is, one more piece of anecdotal evidence.
Gotcha,

Just to make sure I understand what you are saying; let’s pick a distance with 1/10mil SD at 2% of drop; like 900 5.8 mils come up. You can call a Cold Bore in with switching mountain winds on a 1/10- 1.2/10mil target? I don’t normally think in anything but scope units, but what is that? About 3.25” of wind, that’s more than bad ass.
 

Skookum

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May 6, 2017
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Gotcha,

Just to make sure I understand what you are saying; let’s pick a distance with 1/10mil SD at 2% of drop; like 900 5.8 mils come up. You can call a Cold Bore in with switching mountain winds on a 1/10- 1.2/10mil target? I don’t normally think in anything but scope units, but what is that? About 3.25” of wind, that’s more than bad ass.
You are looking at it wrong. It isn't about any one single shot. It is about the average of many shots and increasing over all hit probability. The conditions of every single shot are recorded and all plotted on grid paper and analyzed for trends.

I also noticed how you chose an extremely flat shooting cartridge at relatively close range to make your point. With that cartridge, at that range, it most likely wouldn't matter.

But, if you miss off the edge, just call it whatever you like. Then you don't have to think about it too hard. If you don't believe it, then don't use it. Go on doing nothing and be happy with doing nothing.
 
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damoncali

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99% of rifles can't hold .1 mil when fired from a test fixture in a tunnel. There's no way anyone can see that on an open range. Doesn't mean it's not there, but the uncertainty of trying to measure it like that is enormous.
 
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Feb 7, 2013
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You are looking at it wrong. It isn't about any one single shot. It is about the average of many shots and increasing over all hit probability. The conditions of every single shot are recorded and all plotted on grid paper and analyzed for trends.

I also noticed how you chose an extremely flat shooting cartridge at relatively close range to make your point. With that cartridge, at that range, it most likely wouldn't matter.

But, if you miss off the edge, just call it whatever you like. Then you don't have to think about it too hard. If you don't believe it, then don't use it. Go on doing nothing and be happy with doing nothing.
I do not think I am looking at it wrong --

Like you apparently, I have been shooting ELR for long time.

If one says they can "see" and out wind call SD, it would have to be the first shot. The second shot is a correction and ALWAYS has everything in it. Form there, we are ALL just looking at the variables and similarities in wind from the previous shots. I am not saying that SD/AJ/CE/EV are not real, just usually hidden in other variables.

In ELR, hit percentages go way up when the correction is clear and ultra fast, and short. Up .2 / Left .3, whatever, or the actual new setting might be the only things said without math just using the ruler..

Hell, I am not even saying your not as good as the best wind callers in the world, but I have yet to see one that really can out call SD in switchy wind. It is a very tall statement to say that one is so good they can see the actual value and then out call SD. Sure lots of us have said for years, the the model seems way too aggressive, but the problem is even over thousands of ELR rounds, we still can't pin point the "exact" amount. OK, Jim B's 2% seems to make sense, more so than any Ballistician's offerings, but even then, it is very hard to pull out of the real wind.. That's why we need smart guys like him :).

"…all models are approximations. Essentially, all models are wrong, but some are useful." — George E.P. Box, Norman R. Draper

Reminds me when some people were running around here and saying they "had to account for CE" because they were such great shots. Obviously most is hidden in velocity variations and wind; It was more of an ego exorcise than anything else.

Sure put the values in, but no need to focus so much energy on it.
 
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Dec 4, 2002
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Does anybody use a vertical correction for horizontal wind?
Is there a program that takes it into account, or like SD varies a lot depending on the bullet used, twist, etc.?
Theoretically, there is an up/down component, but I never see people using it.
 
Jun 26, 2012
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Does anybody use a vertical correction for horizontal wind?
Is there a program that takes it into account, or like SD varies a lot depending on the bullet used, twist, etc.?
Theoretically, there is an up/down component, but I never see people using it.
its called aerodynamic jump and its usually built in to the solution. i know AB uses it for sure.
 
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Correct... http://bulletin.accurateshooter.com/tag/aerodynamic-jump/

But since we are discussing SD, aerodynamic jump has basically the same problem: it does exists, but establishing exactly (not guessing) what angle of attack the bullet takes in flight is the core of the problem.

The fact that the wind is often variable (in speed and direction) across the bullet path further compounds the problem.

That, and the need for a quick correction (even if is not that accurate) makes adding up all the tech tid bits difficult until the day we can have real time wind reading downrange, at several points, integrated into a program for a really accurate ballistic solution.
 

Skookum

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First, I don't usually make follow up corrections. Virtually every shot I take at ELR distances is in a different wind condition. While practicing, I have no interest in hitting the target with the second shot.

Second, I find it extremely incongruous that when presented as a part of a ballistic program, people will happily use the SD value included. However, if it is suggested that a similar number might be calculated and figured in manually then it is automatically bullshit and useless.
 
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Correct... http://bulletin.accurateshooter.com/tag/aerodynamic-jump/

But since we are discussing SD, aerodynamic jump has basically the same problem: it does exists, but establishing exactly (not guessing) what angle of attack the bullet takes in flight is the core of the problem.

The fact that the wind is often variable (in speed and direction) across the bullet path further compounds the problem.

That, and the need for a quick correction (even if is not that accurate) makes adding up all the tech tid bits difficult until the day we can have real time wind reading downrange, at several points, integrated into a program for a really accurate ballistic solution.
It’s an impossible variable to calculate for the reasons you stated. As the bullet slows yaw of repose changes which change sAj compounded by a slight difference in wind speed and relative wind blah blah blah. Best you can get as an average of a distance and use it. Just like SD and CE, it all gets baked into the actual drop/wind hold. You get that by actually shooting. If it takes me 5.6 mils to hit at 800, I honestly don’t care how much is AJ and how much is gravity. It’s 5.6 either way. If I was building a solution by hand and trying to get first round hits then yeah I would care. It’s all good topics of discussion but we are all getting really really deep in the Woods here. Shoot. Record dope. Tweak calculator to match dope. Have fun.
 
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First, I don't usually make follow up corrections. Virtually every shot I take at ELR distances is in a different wind condition. While practicing, I have no interest in hitting the target with the second shot.

Second, I find it extremely incongruous that when presented as a part of a ballistic program, people will happily use it. However, if it is suggested that a similar number might be calculated and figured in manually then it is automatically bullshit and useless.
I didn't see anyone, including me say that it is untrue or bullshit. Hell, I like Jim's 2% idea, it seems to be much closer as I said to what we have seen.

The laser focus on SD is not going to make us hit more, that is the point. Most I know, brake it down in very rough distance windows like 1K,1500, 2K, 2500 manually, but none, are doing SD math for every distance and shot. None, claim they can call wind so well that can spot Eötvös, Coriolis, AJ, and SD apart from from what the wind, internal ballistics (SD/ES) and optical disturbances as an aggregate, are doing.

What is the low angle sun doing to the target image? Particles in the air at ELR distances? How about, Mirage? These optical disturbances can be many times the SD or CE, but there not as sexy to talk about..

I'm sorry, but while yes, the wind is ever changing, the idea that the second shot is somehow Cold Bore; is off; You have info, including the shooters offset now included in you next. Unless you are always drastically changing your FFP for each new shot.
 
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Skookum

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I didn't see anyone, including me say that it is untrue or bullshit. Hell, I like Jim's 2% idea, it seems to be much closer as I said to what we have seen.

The laser focus on SD is not going to make us hit more, that is the point. Most I know, brake it down in very rough distance windows like 1K,1500, 2K, 2500 manually, but none, are doing SD math for every distance and shot. None, claim they can call wind so well that can spot Eötvös, Coriolis, AJ, and SD apart from from what the wind, internal ballistics (SD/ES) and optical disturbances as an aggregate, are doing.

What is the low angle sun doing to the target image? Particles in the air at ELR distances? How about, Mirage? These optical disturbances can be many times the SD or CE, but there not as sexy to talk about..

I'm sorry, but while yes, the wind is ever changing, the idea that the second shot is somehow Cold Bore; is off; You have info, including the shooters offset now included in you next. Unless you are always drastically changing your FFP for each new shot.
None of these factors are to be considered in isolation. The others haven't been mentioned because they aren't the subject of the thread.
 
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Dec 4, 2002
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Come'on guys, it's just a friendly discussion...

There are many ways to slice it. Perhaps some better than others, but precisely we're here to get/share knowledge about all these tech tid bits, and see at what range, and in what case caring about secondary effects pays dividends.
 

lash

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Correct... http://bulletin.accurateshooter.com/tag/aerodynamic-jump/

But since we are discussing SD, aerodynamic jump has basically the same problem: it does exists, but establishing exactly (not guessing) what angle of attack the bullet takes in flight is the core of the problem.

The fact that the wind is often variable (in speed and direction) across the bullet path further compounds the problem.

That, and the need for a quick correction (even if is not that accurate) makes adding up all the tech tid bits difficult until the day we can have real time wind reading downrange, at several points, integrated into a program for a really accurate ballistic solution.
Jim Boatright addressed AJ in one of his articles in the Resource section. If I recall correctly, he stated, with backup calculations, that AJ takes place only once in a shot, and that is immediately after the projectile first leaves the barrel. That would make the wind at the shooter the only one that matters for that calculation. Thus why it can be included in ballistic applications.

Now, that does not mean that crosswinds at distance have no effect at all, but those effects are not aerodynamic jump.

Here, from your included linked article:
 
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Hi,

@TiroFijo my question in regards to the link you posted would be for its' author...

Which is it???

"However, the horizontal and vertical drifts due to the yaw of repose cannot be calculated because the necessary a very expensive laboratory instrumentation."

"Luckily, the drifts are small compared to the deflections."

OR

"But, the drifts can be observed by skilled target shooters."

So he states that it would take very expensive lab instruments to calculate the drifts BUT the drifts CAN be observed by skilled target shooters :)
Appears he doesn't really want to make a decisive decision on the subject......

Sincerely,
Theis
 
Feb 20, 2013
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True, with a constant wind.

But the angle of repose of the bullet changes with a changing wind...

http://www.exteriorballistics.com/ebexplained/articles/article2.pdf

"the horizontal and vertical drifts due to the yaw of repose cannot be calculated because the necessary a very expensive laboratory instrumentation. Luckily, the drifts are small compared to th deflections."
Exactly. And there is no other possible way unless Newton's laws (the first one in particular) are wrong...which are not thank God!:)
 
Dec 4, 2002
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Both things can be true at the same time... calculating secundary effects such as SD and vertical POI variations wiht a crosswind, as you well know, require measuring certain bullet parameters with extensive and expensive testing.

Shooters could observe these effects at certain ranges/conditions (that's what we are discussing here). To identify, isolate and measure them is another thing. Most errors are just buried under shooter/wind/who knows? error.

I think Bill McDonald estimated these effects at long range and came under the impression that a good shooter could "see" them, even without understanding WTF happened. Sadly he passed away a year ago, or I would ask him.
 
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Nov 27, 2017
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Hi,

Here is my take on Spin Drift..may get long but I will try and keep it brief.

1. Does it exist? Yes (I do not think anyone in this thread is stating otherwise)
2. Does it "need" to be accounted for? (My take below)
A. I am on the side that thinks the better you are at wind calling, the less accounting of SD you intend to partake in when considering normal sized targets at ELR distances.
B. I am also on the side that thinks those great wind calling shooters that are shooting smaller targets at the ELR distances tend to utilize a ballistics program that has allowed them to customize their data; whether they have encased their SD into the wind data that is on them.
3. Do I think SD takes priority over wind reading? Absolutely NOT.
4. Do I think SD takes priority over getting your muzzle velocities within 3fps or under for any normal sized target at 2500+m? Absolutely NOT.
5. Do I think taking SD into account will help a moderate wind caller get on target faster? Yes because by partaking in the science of SD and accounting for it (EVEN IF MENTAL accountability) could allow that moderate wind caller to reduce their allowed error budget numbers; which puts them on target. The GREAT wind caller does not need this help in his error budget as he reduces his error budget by larger percentage by getting his wind calls closer to real wind measurements.

So IMO the small reduction in error budget by accounting for SD is better suited to moderate wind callers with normal sized targets OR great wind callers with relative small sized targets.

In reality....this is one of those things that are akin to asking 3 K9 trainers the best way to teach the "out" command....the only thing you will ever get any 2 of the 3 to agree on is that the other is wrong.

So IMO part 2...SD is one of those things that have been talked about so much (with no agreement and/or conclusion) by companies within the shooting industry that the industry itself has just looked at it as marketing hype and hoopla. With no intentions to prove this vs that.
It is viewed as "It is what you want it to be" most of the time but with equipment and testing facilities FINALLY being sought after by those in the industry I think we will see some agreements and conclusions being made in the very near future.

Sincerely,
Theis
 

Skookum

Knuckle Dragger
May 6, 2017
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A. I am on the side that thinks the better you are at wind calling, the less accounting of SD you intend to partake in when considering normal sized targets at ELR distances.

5. Do I think taking SD into account will help a moderate wind caller get on target faster? Yes because by partaking in the science of SD and accounting for it (EVEN IF MENTAL accountability) could allow that moderate wind caller to reduce their allowed error budget numbers; which puts them on target. The GREAT wind caller does not need this help in his error budget as he reduces his error budget by larger percentage by getting his wind calls closer to real wind measurements.

So IMO the small reduction in error budget by accounting for SD is better suited to moderate wind callers with normal sized targets OR great wind callers with relative small sized targets.


Sincerely,
Theis
This makes absolutely no sense to me the way you wrote it. A moderate or poor wind caller would only benefit if the wind was from the right and SD canceled out some wind he had under accounted for. Most new wind callers under estimate wind anyway. It would compound the error though, if the wind was from the left.

An expert wind caller might be well within his error budget without SD, but if the value was discoverable, why would he not be interested in increasing the error budget by applying the correction?

At 1000 yards and less, all of this is minutea. At a mile, using the 2% of drop figure just for a number, my spin drift is 0.3 mils. That is half of the 36" plate. At 2350 yds it is more than the width of the 36" target. The wind call could be absolutely perfect and you would still have about a 50% chance of missing the target depending on where that round fell within the dispersion of the group.

Where am I going wrong here with this logic?
 
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Feb 7, 2013
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This makes absolutely no sense to me the way you wrote it. A moderate or poor wind caller would only benefit if the wind was from the right and SD canceled out some wind he had under accounted for. Most new wind callers under estimate wind anyway. It would compound the error though, if the wind was from the left.

An expert wind caller might be well within his error budget without SD, but if the value was discoverable, why would he not be interested in increasing the error budget by applying the correction?

At 1000 yards and less, all of this is minutea. At a mile, using the 2% of drop figure just for a number, my spin drift is 0.3 mils. That is half of the 36" plate. At 2350 yds it is more than the width of the 36" target. The wind call could be absolutely perfect and you would still have about a 50% chance of missing the target depending on where that round fell within the dispersion of the group.

Where am I going wrong here with this logic?
Your not. Before the 2% thing, many as I mentioned, used brackets (1k=.1L, 1.5k=.2L, 2k=.3L, 2.5=.4L) where we considered about 50% of the solvers prediction— the two methods line up scary closely. Honestly, a 1-2mph wind can still hide whats truly happening, much less those big days.

But a few solvers including AB Analytics seem to have recently softened the SD predictions too.
 
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Nov 27, 2017
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Hi,

If your SD accounts for half your target size then you probably need a better cartridge :)

We are in the process of setting up a ballistics testing facility that has weather stations every 400m out to 3600m with multiple Weibel Doppler Radars and so we will be able to get the positional data of projectiles in flight and compare their real time position to "calculated" positions.
This will allow us to put a more firm finger on the science of things because lets face it..without over 10k rounds of each projectile in question paired with each twist rate in question sent down range at appropriate testing facilities everything is still up in the air.

These type of conversations always get into someone attempting to prove right vs wrong instead of proving the how, when and why of the conversations. Kinda like when the original Cheytac and Associates released the ballistics data on the 419gr .408.....interwebs called BS on its' BC actually increasing at the approximately 800m mark even though they had radar test to prove it. NOW fast forward decades and we have Jims "Coning Theory" which puts the science theory into the conversation.

Sincerely,
Theis
 
Feb 7, 2013
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Hi,

If your SD accounts for half your target size then you probably need a better cartridge :)

We are in the process of setting up a ballistics testing facility that has weather stations every 400m out to 3600m with multiple Weibel Doppler Radars and so we will be able to get the positional data of projectiles in flight and compare their real time position to "calculated" positions.
This will allow us to put a more firm finger on the science of things because lets face it..without over 10k rounds of each projectile in question paired with each twist rate in question sent down range at appropriate testing facilities everything is still up in the air.

These type of conversations always get into someone attempting to prove right vs wrong instead of proving the how, when and why of the conversations. Kinda like when the original Cheytac and Associates released the ballistics data on the 419gr .408.....interwebs called BS on its' BC actually increasing at the approximately 800m mark even though they had radar test to prove it. NOW fast forward decades and we have Jims "Coning Theory" which puts the science theory into the conversation.

Sincerely,
Theis

Keep us posted, sounds great-- Any chance of getting the funding to add wind Ladar?
 

Jim Boatright

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Feb 21, 2018
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True, with a constant wind.

But the angle of repose of the bullet changes with a changing wind...

http://www.exteriorballistics.com/ebexplained/articles/article2.pdf

"the horizontal and vertical drifts due to the yaw of repose cannot be calculated because the necessary a very expensive laboratory instrumentation. Luckily, the drifts are small compared to th deflections."
The yaw-of-repose of a spin-stabilized projectile in flat firing is a strictly horizontal attitude bias of the coning axis pointing direction, always perpendicular to the gravity gradient which is its cause. Its vertical component is identically zero at all times. The yaw-of-repose is also completely independent of any ambient winds in the atmosphere through which that projectile is flying.
 

Jim Boatright

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Jim Boatright addressed AJ in one of his articles in the Resource section. If I recall correctly, he stated, with backup calculations, that AJ takes place only once in a shot, and that is immediately after the projectile first leaves the barrel. That would make the wind at the shooter the only one that matters for that calculation. Thus why it can be included in ballistic applications.

Now, that does not mean that crosswinds at distance have no effect at all, but those effects are not aerodynamic jump.

Here, from your included linked article:

Thanks for pointing this out, Lash. Yes, the CWAJ trajectory deflection is a one-time transient effect which depends upon the (presumed horizontal) crosswind first encountered by the bullet after it exits the muzzle blast cloud (shockwave) a few yards ahead of the muzzle. Gustavo and I expect that the wind speed and direction can be measured at the firing point. This vertical angular deflection of the entire remaining trajectory can be compensated by a suitable elevation aiming correction. It really has nothing to do with the remainder of the windage estimation problem.
 

damoncali

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Drift and jump have nothing to do with each other. Drift is due to a yaw of repose, which is defined as a quasi-steady state yaw (basically, the average yaw) to the side caused by gravity and spin. That yaw is relative to the airflow, which already accounts for wind. YOR imparts a lift force, which causes drift. Wind has nothing to do with it. The angular value grows with distance because the lift force is always present.

Wind deflection is caused by drag, not lift. Drop is obviously caused by gravity, and slowed by drag. This is why wind and drop are related, and both are covered by BC.

Aerodynamic Jump is due to an angular misalignment and/or rotational velocity upon exiting the bore. It is a transient effect that happens immediately after launch, and it causes an angular change in the line of departure. The only thing it has to do with wind is that wind can be the source of the initial tipping.

CG Jump is similar to AJ, but it results from a lateral offset CG of the bullet (due to manufacturing tolerances). Think about the direction a CG is moving when it leaves the bore. If it's offset from the centerline, it will leave at an angle determined by its helical path down the bore. Wind plays no part here, but your bullets do. This is one reason FMJs suck (AJ is another).

Neither Aerodynamic Jump nor CG Jump grow angularly with distance. There is no down range force that causes those effects.

They can all be calculated if you have good bullet aerodynamic data. The calculations are not hard, but gathering the data is.
 
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Feb 20, 2013
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Drift and jump have nothing to do with each other. Drift is due to a yaw of repose, which is defined as a quasi-steady state yaw (basically, the average yaw) to the side caused by gravity and spin. .
Isn't YOR caused by the overturning moment? Gravity OK but spin not so, AFAIK.
 
Feb 7, 2013
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I am so confused.

After this discussion this is my new game plan for a longish shot in my next positional match. Hope I got this correct. Flight plan is much further than the LOS distance due to circling around the target. I think I can miss-right but circle back around with a left twist.

I’ll also call the wind as dial left/right hold for a center impact
4759AFDF-EBA0-4071-81EE-8580F9263132.jpeg

Sorry at my white board and board while the engineers debate something I don’t understand at work.
 
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Jim Boatright

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Isn't YOR caused by the overturning moment? Gravity OK but spin not so, AFAIK.
The spin-stabilized bullet's yaw-of-repose (beta subscript R in my papers) is caused by two small overturning moment TORQUE IMPULSES per coning cycle, centered on the Top Dead Center and Bottom Dead Center bullet positions in that coning cycle. During each half coning cycle, the apparent wind direction seen by the flying bullet keeps dropping below the nose of the bullet as its trajectory arcs evermore downward due to gravity. For a right-hand spinning bullet, with its angular momentum vector pointing forward along its spin-axis, each of these torque impulses tugs that angular momentum vector (and hence the nose of the bullet)) slightly rightward in flight. The accumulation of this reinforcing sequence of small rightward attitude changes produces the yaw-of-repose at each point during the flight.

Here, we use the term "yaw" in the sense of an "aircraft type" sideslip attitude angle, as opposed to an "aeroballistic yaw" which we generally use to define the total aerodynamic angle of attack for rotationally symmetric projectiles. That aeroballistic yaw is the vector sum of the aircraft-type pitch and yaw attitudes as small, non-Eulerian attitude angles.

In Coning Theory, we see that the "yaw-of-repose" is the (aircraft-type, horizontal) yaw attitude of the coning axis itself. Because the coning motion of the CG of the bullet is symmetric about this coning axis, this yaw-of-repose is also the "average" yaw attitude of the bullet's spin-axis, as correctly described above by Damon.
 
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Jim Boatright

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If you guys have "Rifle Accuracy Facts" by H. R. Vaughn, take a look at "wind drift" from page 195 on.
Yes, Tiro, Harry Vaughn had the CWAJ effect about right, but not having Coning Theory, he did not explain it quite correctly. I had to buy a second copy of his wonderful book about 10 years ago because I had worn out my first one. Bob McCoy also explained CWAJ and almost discovered Coning Theory in that process. The extreme examples of CWAJ which come to mind would be firing sideways from a flying aircraft as with the waist gunners in a B-17 or with an AC-130 gunship.

The "skew angle" concept of accounting for CWAJ is only useful in Known Distance (KD) target shooting, particularly in 50-yard rimfire competition. The skew angle "flattens out" significantly at longer ranges. The horizontal windage correction varies approximately with the square of range, while the CWAJ remains a fixed elevation angle correction at all ranges.
 
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