Dissimilar Wind Drift Question for Bryan

[Scott Harris]

Bryan,

You say the effect is spin drift by another name, but I'm pretty sure David is claiming an opposite effect: more drift in a RIGHT wind and less drift in a LEFT wind for a right-twist barrel. See here: http://www.zediker.com/DTR_PDF_links/DTR_DWD_article_one_2013.pdf

Also, if you were going to test this theory, wouldn't you start with rifles that have 1000 yard no-wind zeros...thereby eliminating spin-drift as a factor? We need you to come out West and test this sir! Or send me a pair of barrel blanks (7mm 9-twist ) and I'll get them chambered and tested. I'll get my Phoenix F-Open buddies to do it on the 1K line at Ben Avery. It might take a few shooting sessions in different conditions to get solid 1K zeros. But, once this is accomplished, it would be short work to prove or disprove this idea.

Thanks,
Scott

 

[dcali]

The direction of the effect isn't really that important because either way it's unexplained and could be confused with spin drift. Which of course, makes it hard to refute. Both articles posted referenced Bob McCoy as having written about the effect. If that paper can be produced, perhaps some light could be shed on the topic.

 

[Berger.Fan222]

I don't see how referring to dissimilar wind drift as sophistry or impugning motives by referring to a patent application or invoking arguments by authority contribute to the conversation. Nor do I see how shifting the burden for proponents of DWD to produce a McCoy paper as productive when McCoy's references to DWD as "lateral drafting" may have been in presentations or private communication.

Further, McCoy's original work on wind drift is over 40 years old, was only purported to be more accurate than riflemen could estimate wind across the range distance, and critical tests with right and left handed barrels may have never been performed or failed to make it into print. Could a 12% effect have been missed? Can anyone cite data showing equal magnitude drifts (< 12% uncertainty) in left and right twist barrels?

The hypothesis that Tubb is confounding DWD with spin drift is completely obliterated by the fact that the effects are in opposite directions, unless one further hypothesizes that Tubb can't tell the difference.

The effect may be "hard to refute" for keyboard commandos, but some rather simple experiments have been proposed. No one has suggested why shooting (otherwise identical) left and right twist barrels that are sited at 1000 yards in no wind conditions and then comparing their wind drift once a significant wind begins to blow fails to represent a valid test of the hypothesis. Tubb's prediction seems clear enough: in a right to left wind, the bullet from the right twist barrel will drift more. Sighting in the rifles under no wind conditions eliminates spin drift as a confounding factor.

Reading Tubb's papers indicates he has ample experimental data to support his hypothesis. Attempts to answer hard data from direct observations with theoretical arguments seems sophomoric, especially when combined with the rhetorical attempts to impugn Mr. Tubb's character. When a man of Mr. Tubb's accomplishments joins us in a forum discussion, I would hope for him to be treated better.

 

[dcali]

Let me be clear. In no way am I suggesting that Mr. Tubb is doing anything deceptive, unethical or otherwise in bad character. I just think he's wrong about this. And I would love to read McCoy's work on the topic, but I cannot find it (not an uncommon thing to happen, as many of these papers are old and were not well distributed).

 

[savagedasher]

This may or may not be helpful whatsoever,but ...

As a 1000 yard benchrest shooter, I have noted a few things. I always write down the condition as I saw it before leaving the line. I can tell you on deep creek range, and only there do I have any experience. Whenever I wrote left to right, my groups shape with very little vertical, but are almost always wider than I would hope for. The exact opposite when it's right to left. I think I am seeing aerodynamic jump to some extent, but also just topography of the range.

I can't see how to even begin to test something like this, it would take an indoor 1k range. Fan's, perfectly symmetric building and so on.

Tom

Tom that is the same pattern I find working with a tuner. Saturday shooting I had a big gun on my right. he was getting both vertical and horizontal in the 20 MPH head wind . My Dasher had way less BC. but wasn't effected near as much by the wind. My ES were under 5 when I tested the day before. It was also tuned and shooting in the .150 or less at 100. Paper test said the bullet was asleep at 50yds. Larry

 

[Bryan Litz Ballistics]

I was able to get the link to work (for the DTR testing article) printed and read it.

The paper cleared up a misunderstanding that I had. I thought that dissimilar wind drift (DWD) acted in the same direction as spin drift. The article clearly states the opposite.

Based on this correction, I'll have to modify my hypothesis of DWD:

Hypothesis:
What's been identified as DWD is actually the error between predicted spin drift and actual spin drift.

Remember, this is just a hypothesis. Let me explain.

The Tubb article states that DWD was discovered because "... the lateral drift of the bullet was not the same depending on the wind direction."

This observation was made using a reticle which 'accounts for spin drift'.

At this point I'll take a little detour and discuss the nature of spin drift.

Formulas used to calculate spin drift are approximate. The most commonly used spin drift formula is one I published in "Applied Ballistics for Long Range Shooting". The spin drift formula requires the gyroscopic stability factor (SG) as an input. The most common formula used to calculate SG is also an approximation. So the most common spin drift formula is an approximation based on an approximation.

Why use approximations? Short version; it makes a solution practical. If you were to calculate SG and SD in their fundamental form, you would need to know the bullets complete mass properties including axial and transverse moments of inertia; complete aerodynamic properties including pitching moment coefficient and dynamic damping derivatives, and many other obscure things. These are some of the most difficult aero coefficients to measure and predict for spin stabilized bullets, so even if you were to solve the full equations, the answers would still not be exact.

The approximate solutions for SG and spin drift are based on sound math, and they're much better than nothing. However they're far from perfect.

An example from my own spin drift testing.

We built two 308 Win rifles, similar in every way with exception of left vs. right twist barrels. We volley fired into a common point of impact 32" above the point of aim at 100 yards on a plumb line. This 100 yard volley fire insured the rifles were zeroed together, and the only difference in their point of impact at long range would be the difference in spin drift between the left and right twist barrels.

Two bullet types were tested.

175 grain Berger OTM
When 10-shot groups were fired from the two rifles at 1000 yards, there was 22.8" horizontal difference between the group centers. That means that each group drifted 11.4" from center. The calculated spin drift for this bullet in the conditions of the day was 11.2". Considering the group size and minor data scatter, this is a pretty close estimate for the calculation.

185 grain Berger OTM (Juggernaut)
Same procedure was employed for this bullet. In this case, the 1000-yard group centers were 6.7" left and right of center but the calculated spin drift was 8.8". So in this case, the formula missed the measured value by 2.1 inches. Wind was very low for both of these tests.

Full details of this test are published in Modern Advancements in Long Range Shooting - Vol 1, Chapter 12.

To wrap up our spin drift detour, I can say that the formulas available for calculating spin drift are approximations, and do not always agree perfectly with direct measurements.

Now, back to the DTR reticle.

Recall that DWD was observed as follows: "... the lateral drift of the bullet was not the same depending on the wind direction."

This observation was made using a reticle which 'accounts for spin drift'.

What do you suppose would happen if the original reticle which 'accounts for spin drift' were not perfectly accounting for spin drift? The formula for calculating spin drift is not exact to begin with, but when it's etched in a reticle, how can you be sure it's right for any bullet, twist, MV, and DA that you shoot thru it? Suffice it to say, it's likely that the baseline spin drift hold was not perfectly accurate to begin with, in which case, one would observe the error between calculated and actual spin drift. This error is what's being described as dissimilar wind drift.

In summary, how do you know that DWD is not just accounting for the error between predicted and actual spin drift?

Also, I'd be very interested in reading the material that David Tubb cited from Bob McCoy referencing 'lateral drifting'. David, can you produce this source that you cited in your article? We're interested and no-one can seem to find it.

-Bryan

 

[lawman29]

Here's a book by a Robert McCoy maybe it's in there somwhere:
http://www.amazon.com/gp/product/0764307207/ref=as_li_ss_tl?ie=UTF8&tag=bison0b-20&linkCode=as2&camp=217145&creative=399349&creativeASIN=0764307207

 

[dcali]

It's not in there. But Bob McCoy wrote a metric ton of papers besides that excellent book. Unfortunately, they're hard to find unless they made their way onto one of the big online databases.

By the way, to anyone who might be tempted to buy Modern Exterior Ballistics, get the 2ND EDITION! The first is so full of typos that it will make your head spin.