Wind drift question.

[450BM]

I pulled this from this article: https://hardairmagazine.com/ham-columns/the-external-ballistics-of-slugs-in-airguns/

The drift was calculated using ChairGun. Note that the least wind drift occurs when starting with a muzzle velocity of about 900 fps, and increases when the MV is either slower or faster.

The wind drift at an MV of 1500 fps is about 50% more than at 900 fps. In fact it is as poor at 1500 fps as it is at about 600 fps for most slugs we use. You have to use an MV of about 2500 fps before the wind drift is as low as what it is at 900 fps!

This chart is quite a shock to most slug shooters. It shows an increase in drift of about 10% by increasing the MV from 1000 fps to 1100, and an even greater increase above that. Combined with how much more air it takes to reach 1100 fps, if shows that even for slugs there is little point striving for much over 1000 fps.

It looks like ChairGun‘s charts are screwed up and the author assumed they were correct. If this has been discussed before could someone please direct me to the conversation because this makes no sense to me at all…

 

[dgeesaman]

Wind drift is directly derived from the difference in time of flight between a theoretical projectile with constant velocity and actual projectile that does lose velocity. Ballistic coefficient is the parameter that tells us how much a real projectile slows down from constant velocity. I’m trusting Brian Litz on this.

So the weird wind drift data you’re seeing might be related to the BC models and data in the transonic zone. Bc changes quite drastically on both sides of the speed of sound. ChairGun might use stepwise values of BC based on velocity, which can give weird (stepwise) results. Hardly smooth but more accurate than a constant bc.

 

[450BM]

Wind drift is directly derived from the difference in time of flight between a theoretical projectile with constant velocity and actual projectile that does lose velocity. Ballistic coefficient is the parameter that tells us how much a real projectile slows down from constant velocity. I’m trusting Brian Litz on this.

So the weird wind drift data you’re seeing might be related to the BC models and data in the transonic zone. Bc changes quite drastically on both sides of the speed of sound. ChairGun might use stepwise values of BC based on velocity, which can give weird (stepwise) results. Hardly smooth but more accurate than a constant bc.

Standard velocity 22 LR is more accurate than high velocity stuff. When HV stuff drops below the sos it wobbles and loses accuracy. I’ve never shot any HV 22 ammo, no matter the price, that would outshoot even my cheapest SV ammo. But, not talking about accuracy here, the author is specifically talking about wind drift. How is it even remotely possible that a bullet or slug or anything, fired at 2500 fps MV would drift more than something with a MV of 900 fps.

The article was written in July 2019 but there’s no comment section to question it. Just tried their “Contact us” link and their “Submit” button doesn’t work.

 

[Ballisticboy]

The effect is caused by the increase in projectile aerodynamic drag coefficient. At speeds between about 1000ft/sec and 1200ft/sec there is a sudden and very large increase in the drag coefficient, roughly doubling in size, usually referred to as the transonic drag rise. The exact speed range depends on the shape of the projectiles.

As speeds increase beyond the above speeds, the drag coefficient may continue to increase, for very blunt shapes, or it may start to fall again for more streamlined shapes. The drag coefficient is important as it governs the lag time, as described by @dgeesaman above, much more accurately than a BC value.

 

[dgeesaman]

I don’t have the background of BB, so I had to take the time to look up more and sure enough this is not just a quirk in the ChairGun BC algorithms.

Here is a paper on 9mm handgun ballistics, which happen to be very suitable for comparison to a theoretical slug moving under/through/above the SoS. https://apps.dtic.mil/sti/pdfs/ADA607593.pdf
From this, note the drastic change in BC (CD) at Mach 1.1 (attached).

I also plugged in .25 cal, .10 G1 BC, 10mph crosswind, 200y range, 59F / 78%RH to develop this chart (attached), which basically confirms the saddle in wind drift is below SoS. ( Edited: fixed my data entry error in Ballistic AE and added data from Lapuas program.)

I suspect that if we dig a little more we can find additional support for this. The Rimfire PRS shooters get quite deep into this kind of thing and unlike slugs, their velocities actually do go well above SoS sometimes.

 

[dgeesaman]

It would be interesting to fire airgun slugs at supersonic speeds somehow and use a Doppler chronograph to investigate the ballistics of the trans- and supersonic zones. 9mm is good data but it’s not quite the same design as a slug.

Perhaps helium could do it but I don’t believe in wasting precious helium on airgunning. Maybe a carefully constructed firearm could do it but that’s outside of this conversation and forum.

Finally all of this ignores two valuable factors: absolute accuracy and range estimation. Shooting subsonic might cause less wind drift but if the circumstances of your airgunning application cause mostly vertical misses from range error then a flatter trajectory could be preferable. The absolute accuracy would likely boil down to pure scatter and bc variances, both of which require testing to sort out and my gut tells me they are also very important.

 

[Ballisticboy]

Finally all of this ignores two valuable factors: absolute accuracy and range estimation. Shooting subsonic might cause less wind drift but if the circumstances of your airgunning application cause mostly vertical misses from range error then a flatter trajectory could be preferable. The absolute accuracy would likely boil down to pure scatter and bc variances, both of which require testing to sort out and my gut tells me they are also very important.

The problem with vertical misses from range errors, is that in a cross wind it will be complicated more by the vertical error from a cross wind. The greater the gyroscopic stability factor, the larger the vertical error tends to be.

 

[JungleShooter]

Yeah, I would think ranging errors can be kept pretty small, with laser range finder precision being ±1y and under 100y even ±0.5y.

Of course, if we're talking going out to 300y, or shooting a 25gr MRD at only 500fps MV — then even just one yard will cause you to miss the killzone.


Thanks, Miles, for reminding is that wind also messes with our elevation, not just with our windage.
Glad to hear from you again.

Matthias