Wind drift question

[BBerguson]

I copied this from an article discussing wind drift when shooting bullets/slugs from an air rifle:

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 makes zero sense to me. Does it have anything to do with the angle of the shot. I know shooting at an angle up or down reduces the amount of drop. Does it also affect wind drift?

 

[Petrol & Powder]

It makes zero sense to me as well.

I think the writer may have used the wrong words, or there was a typo.

Wind drift is a function of how long the wind acts on the projectile. What vector the wind acts upon the projectile. And the strength of the wind acting on the projectile.

Assuming the range is constant, a faster projectile will reach the target sooner than a slower projectile. Less time traveling to the target equals less time for the wind to act upon the projectile. Therefore, with all else equal, a faster projectile will experience less wind drift than a slower projectile over the same distance.

An NRA Shooting Sports Journal | What Exactly Is Wind Drift?

Having problems with the wind? Crosswinds have plagued shooters for generations, our explanation will keep you on target

www.ssusa.org

 

[BBerguson]

Reading the paragraph, I can’t figure out what the typo or wrong words could be. Maybe he was drunk when he wrote this or just full of poo…

You described just as I have always believed it to be. I can’t believe that shooting an air rifle changes the law of physics. I’ll have to check the forum where this article was posted and see if anyone questions him on this.

 

[BBerguson]

Here is the entire article: https://hardairmagazine.com/ham-columns/the-external-ballistics-of-slugs-in-airguns/

The External Ballistics Of Slugs In Airguns​

ByBob Sterne
July 23, 2019

20


Last month I talked about how the velocity and ballistics coefficient affect the trajectory and wind drift when you are using Diabolo (waisted) pellets. This month I will cover the same ideas, but for slugs in airguns, which are often used by today’s modern high powered PCPs.
Actually, I prefer to use the term “bullets” for these projectiles as that’s the technically-correct description. However, modern airgun lingo refers to them as “slugs”. So I will – somewhat reluctantly – use that term.
For the most part slugs in airguns act in a similar manner to pellets. But there are some subtle differences as we will see…

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Trajectory​

The trajectory of a slug is governed by its flight time to the target. The biggest component of that is the muzzle velocity (MV). If you sight your gun with a single zero (about 25 yards), and look at the drop at 100 yards, at different MVs, you will see a relationship like this:

That chart was prepared using ChairGun, using a BC of 0.100, pretty typical for a light PCP slug. The drop decreases as the MV increases. It is roughly half as much with an MV of 1100 fps (9 inches) compared to an MV of 800 fps (18 inches). So faster is better!
The other major factor affecting the trajectory is the Ballistics Coefficient (BC). That is a measure of how your slug slows, compared to the “standard G1 drag model”.
If the slug slows more, for a given MV it takes longer to reach the target, so the drop is greater.
Here is a comparison of various BCs, all starting at 900 fps, and again sighted for a single zero, charting done on ChairGun:

Most slugs we might use will have a BC of 0.100 – 0.200. A low BC for a slug of 0.050, represented by the red line on that chart, would have a drop of about 16 inches.
Doubling the BC to 0.100 (blue line) reduces the drop to just under 14 inches. But after that the reduction in drop becomes much less as you increase the BC.
Doubling the BC again to 0.200 (green line) only reduces the drop to about 12.5 inches, and even a very high BC of 0.400 reduces that only another half an inch to about 12 inches.
This is an important thing to remember!
Slugs in airguns have a far better BC than pellets. But at 100 yards, there is very little difference in the trajectory with differing BCs.
If you are shooting at longer ranges, 200 yards and beyond, then the BC makes a difference, of course. The trajectory, however, is still primarily a result of the velocity.

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Wind Drift​

As with pellets, the lag time is the key to wind drift. This is mostly governed by the BC.
The chart below show how much the wind drift changes as we change the BC. This was calculated using ChairGun, with an MV of 900 fps and a 90 degree crosswind of 10 mph:

Practically speaking, the sideways drift of the slug is inversely proportional to the BC. Double the BC and you get half the drift.
If you compare this to last month’s chart, you will see what a HUGE difference there is between the wind drift of a slug and a pellet.
A slug with a BC of 0.100 will only drift about 4.6 inches in a 10 mph crosswind at 100 yards, and one with a BC of 0.200 about 2.3 inches.
Compare this to a typical round nose pellet with a BC of 0.030, at over 15 inches of drift, even though they all started at 900 fps, and you can see why slugs are so superior for shooting in wind.
You might think that since slugs have much lower drag, it would make sense to shoot them at much higher velocities, including Supersonic. It turns out that is NOT the case. Check out the ChairGun chart below, for an average slug with a BC of 0.100 in the same 10 mph crosswind:

Once again the wind drift at 800 and 900 fps MV is virtually identical at 100 yards, at 4.6 inches.
If we increase the MV to 1000 fps it increases to 5.1 inches (more than it is at 700 fps). And at 1100 fps it increases further to nearly 6 inches.
Yet again, at 900 fps and above (called the Transonic region), the drag of the slug increases quickly, so the lag time increases, and with it the drift. As with pellets, instead of trying to increase the MV, we should look to the BC to decrease the drift.

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The Velocity “Sweet Spot” for Slugs In Airguns.​

Following on with the above facts about how velocity affects the wind drift, there must, therefore, be an optimum velocity for any given slug at any given range, to have the least possible wind drift.
For most of you this will be quite a revelation, but it is absolutely true. The chart below shows the drift in a 10 mph crosswind at 200 yards, for a variety of BCs:

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.
As with pellets, it shows the extreme advantage in terms of a reduction in wind drift from a higher BC, particularly going from a BC of 0.100 to a BC of 0.200 (half the drift).
Likewise it shows the smaller gains to be made doubling the BC again, although at extreme ranges (eg. 500 – 1000 yds) it would be hugely advantageous. For up to 200 yards, the “sweet spot” for the MV of slugs is about 950 – 1000 fps, and perhaps up to 1050 fps for longer ranges.
Next month I will be looking in detail at how the wind at various parts of the range affects the drift at the target. You may be in for a shock!

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PS. Check out HAM’s recent comparison test of shooting slugs and pellets. This shows some experimental data on shooting both types of ammunition using a Huben K1 air rifle. – Editor.

 

[RicinYakima]

Wind drift is a function of the LOSS of velocity. The articles do not correlated BC and dropping through the transonic barrier. As P&P states, all of this depends upon the angle of the wind in comparison to the flight path of the projectile. Go practice in the wind, that will help, not just when it is nice outside.

 

[BBerguson]

Ric, learning how to shoot in the wind wasn’t the purpose of my post. I’m questioning the accuracy of the article and wondering if the laws of physics for projectiles traveling 600 - 1500 fps was different than other velocities… According to this article they are! I know they aren’t. I’m guessing the Chairgun ballistics calculator was giving erroneous (wrong) information and the author of the article didn’t question it.

 

[Snakeoil]

I would have to read the article again after a good night's sleep. What I think he is showing is a given BC scrubs off velocity depending on its speed. Look at the chart. For subsonic speeds, drift decreases with an increase in velocity. Then in the supersonic range drift increases for velocities that will pass thru Trans sonic within 200 yds. Drift then decreases with higher velocities because the bullet stays supersonic. I suspect if the chart was for 300 yds, the increase due to Trans sonic would be longer.

Also, note the the net increase is very small except for the worst BC. If your changed the scale, the curves would look more like straight lines.

 

[Spindrift]

I think the author has a valid point. I'll try to explain why I believe so.
The wind drift is the product of many factors.

1) The amount of effective cross wind, obviously

2) The flight time of the bullet. If you push it faster, you reduce the time under exposure to wind. So, pushing the bullet faster should reduce wind drift, right?

3) The flight characteristics of the bullet, typically described with the mathematical expression G1 BC. The G1 BC is typically presented as a constant. But it is not a constant, in fact it is velocity dependent- particularly in the typical velocity envelope of airguns.
The graph below is from an article on the Berger website.



This graph is for a modern, top- notch LR bullet, and not a cast airgun bullet. But it illustrates the principles of BC variation.

The author's point is, when staying in the subsonic area, the velocity- corrected BC is relatively high. Increasing velocity will reduce flight time, but at the same time decrease the velocity-corrected BC. Thus, the slower projectile has less wind drift, despite slightly longer flight time.

You'll see from the graph that BC is actually quite constant through typical centerfire rifle velocities. But around the Mach1- region, some counter- intuitive things can happen.

 

[RBHarter]

He ran all of this through a calculator not live fire .
BC can be altered some by shape but more weight .
It's not possible to have a 150 gr 50 cal ball with a .3 BC but it's a standard under 30 cal .
A 50 gr wasp waist 25 cal can't get over .18 an RN 100 gr will be real close to .3 if not over , the changes in BC won't be linier .

I remember reading something that made sense in an aerodynamic application but not when applied in the mass acceleration context . Actually it's easier to "see" in hydrodynamics than aero . It has to do with the wave compression changing the the density and the relationship altering the BC .
It explained for me why fast wing shapes aren't until they reach a flow speed it also clears up why you reach a weight to dia point where shape only plays a minor role .

It's possible that the calculator used couldn't account for the weight and BC in caliber . Garbage in garage out .

 

[Petrol & Powder]

My first impression is the author of that article is attempting to appear smarter than he is.

While it is true that different bullet shapes will be affected differently by a crosswind, there are no shapes that are immune to a crosswind. Some spinning projectiles may resist deflection by the wind better than others.

I think that author took some physics and some math and attempted to apply that as pure theory. In the real world, there are more factors. It gets messy.

For starters, there is the cosign of the wind direction. Not all crosswind will be exactly 90 degrees to the bullet’s path. Then if you’re dealing with transonic projectiles that start out supersonic and then drop below the speed of sound, you’re dealing with some more variables. Then we have the portion of the bullet’s flight that was affected by the crosswind; did the crosswind push the bullet early in its flight? Near the end of its flight? Continuously and at the same rate during its flight?

And then there’s gyroscopic precession, which is WAY outside my abilities to intelligently discuss.

While the physics the author cites are factors – they are not the ONLY factors in play. It reminds me of the days before ABS brakes were common. Skid marks produced by a locked up tire could be measured and if you knew the coefficient of friction for the surface; you could calculate the speed needed to produce that length of skid mark. However, that didn’t mean you always knew the correct speed based on the length of the skid marks. A car that struck a concrete bridge abutment that only left 8’ of skid marks before impact wasn’t necessarily travelling slowly due to the short skid marks! There was another factor in play (that immovable bridge abutment). The real world is messy.

 

[L Ross]

I skipped reading all of the charts and graphs but here is what I was told about .22 lr high velocity vs. standard velocity. I do quite a bit of fairly long range .22 shooting, commonly to 300 yards. No one that I shoot with shoots hi-velocity ammo after they try it against standard velocity. The explanation I got as to why wind affects hi-velocity, (above 1,088 fps a sea level), more than ammo below the speed of sound is that the wind has more to "grab" on the HV rounds because it is pushing a bigger "bow wave" of compressed air surrounding the HV bullet and that "bow wave" is much larger on the HV as opposed to the SV. The BC of .22 rimfire bullets are largely the same as loaded. Yet even though they are slower, the SV rounds show less wind drift and that can be proven empirically by shooting at a paper target on a windy day using both types of ammo. Since I can calculate for drop so much easier than wind, I opt for the better "wind bucker."

I started thinking about the bullet and the air it is compressing as a package or a unit instead of just the bullet and that made sense to me. Now a lot of things that "make sense" are often bunk when exposed to true scientific scrutiny, but it is a topic for discussion.

 

[popper]

But around the Mach1- region, some counter- intuitive things can happen. Projectile transiting sonic barrier does funny things. BC doesn't make much difference there. Wind does.

 

[Snakeoil]

I started thinking about the bullet and the air it is compressing as a package or a unit instead of just the bullet and that made sense to me. Now a lot of things that "make sense" are often bunk when exposed to true scientific scrutiny, but it is a topic for discussion.

Yup, the stuff you cannot see, and if you are not aware, envision is normally what lets you come to the wrong conclusion. During a dark period in my career, I was doing statistical analysis of business practices and processes, truly an enormous waste of time due to the human factor. When we reported our findings, very often managers who were told that their operation was costing the company money would push back and spout out numbers and experience and other puffery. When they were done, they would often be told, "You don't know what you don't know.". That's a very powerful statement that normally makes folks take a step back and listen.

I think that is what we have here. I'm not going pretend I understand everything that is in play when a bullet travels downrange. But all the points mentioned here, like BC changing with velocity, the size of the pressure wave being "part" of the bullet, etc., is all stuff that one may not consider. It is very easy to understand a simple relationship like the less time it takes the bullet to get to the target, the less time the wind has to move it. And that relationship applies. It just does not apply all by itself.

At Wilton, we have small, short berms at 50, 100, and 150 yds that we often have to shoot over to hit targets at 200 and 300 yds. Shooters were puzzled by bullets that would impact 6 inches or more, higher than their average group. There were theories about head winds slowing the bullets down and tail winds speeding them up and this was the cause. And it does seem to make sense. Before I started reading tech articles about bullet flight, I thought pretty much the same thing. But as I got better informed, I realized it was those stinking little berms we were shooting over that was taking those head and tail winds and turning them into updrafts and vortices. We still have guys that see the trees on the top of the berm they are shooting at moving with the wind and they think that the bullet is moving at the last moments before hitting the target due to those winds.

I would love to be able to go to DARPA or some other facility where they can actually track a projectile and see what effects its flight and how. Reading about it and forming a mental image is okay. But actually seeing it with high-speed cameras or watching a simulation is so much more fun.

Yesterday was the long range .22 match at Wilton. Targets set at 50, 100, 150 and 200 yds. Wind was present and changing frequently. My partner and I shoot scopes so we have 3 MOA diameter targets. In spite of all the physics in the pellet discussion and the discussion that followed here, bottom line is you have the bullet/ammo you have. What you need to understand is how it will react for your given conditions. You are not going to put up a windspeed indicator. You have flags, you have tall grass and those are your wind speed and direction indicators. My partner and I both call the wind using the same terms. We say no wind, light, medium and heavy. We give direction with a clock, 9 to 3 or 3 to 9. We also try to call any angle by saying something like "3 to 9 with a slight head wind", or "mostly headwind with a slight 3-9". Using that and having many rounds under our collective belts for the various conditions, we have learned how much to either adjust or hold off to compensate. We have also learned how much the wind effect will be at various yardages. Now, I suppose we could go home and plug all the numbers into a ballistic calculator and have it tell us how good or how bad our estimates are while shooting. And maybe that would help. But I suspect that more data would just be more info to process before deciding where to hold and if you get it mixed up, it's a miss. I believe in the KISS principle. Steve and I refer to it as finding the sweet spot. We try to get a scope setting with a center hold that will keep us on the target for most of the varying conditions. Then if the conditions get more severe, we can adjust our hold accordingly. It works well for us. We'll never be 100% sure. You can't see what you can't see, right? Yesterday, Steve called the conditions, and I did my part, and I missed the 200 yd target at 3:00 by a whisker. Had my hold been just a tad more left, it would have meant a perfect score. Steve had two that just nicked the edge of the target. I saw the target move, yet the bullet went a foot right or left. He had held just a whisker better hold for those shots and hit the target. He shot perfect score yesterday. Sometimes, it's just luck.

 

[Ian]

Bullet breaks free of the bow wave past the speed of sound and enters a whole new set of laes regarding drag and BC. In transonic, everything goes to hell, including a huge uptick in bullet flight noise from about 100 fps under to 50 fps past the point of poking into the bow wave. It takes that long for the whole length of the bullet to transition the pressure front. This is the thing that made the rear control surfaces of aircraft temporarily ineffective when attempting the sound barrier and puzzled engineers for quite a time how to design tails that could make the transition without crashing.

 

[popper]

Had a teacher that flew thunderbolt in the war. Big motor and 4 blade prop would make it go super on control surface. He survived a dive pull out and they figured it out. IIRC he flopped the stick around and ended up inverted and decent slowed. Fortunately a bomb run, not strafing.
Going SS, wave develops nice/normal. Slowing down it decomposes very erratically.

 

[Petrol & Powder]

So getting back to the article at hand, it appears the writer was using a ballistics program called "Chairgun".

He plugged some numbers in, got some results out and reported on what the computer spit out.

I agree with what others have pointed out; his input data seems a bit suspect.

While transonic behavior of projectiles is complicated, it's not as complicated as he wants to make it.

 

[Snakeoil]

So getting back to the article at hand, it appears the writer was using a ballistics program called "Chairgun".

He plugged some numbers in, got some results out and reported on what the computer spit out.

I agree with what others have pointed out; his input data seems a bit suspect.

While transonic behavior of projectiles is complicated, it's not as complicated as he wants to make it.

I don't disagree. I think he wanted to write an article and found something that he could write about that probably nobody reading was aware of.

And the 200 yds as a chosen example confuses me. Can a modern air rifle shoot out to 200 yds and have any chance of consistency/accuracy?

 

[Petrol & Powder]

I don't disagree. I think he wanted to write an article and found something that he could write about that probably nobody reading was aware of.

And the 200 yds as a chosen example confuses me. Can a modern air rifle shoot out to 200 yds and have any chance of consistency/accuracy?

Yeah. I got hung up with 200 yards ranges, 1100 fps quotes (muzzle velocity, not down range velocity which would be slower) BC for air gun projectiles that seemed odd and some other stuff. It looks like he cranked some number out and wants to take credit for what the computer spit out.

Yes - projectiles that start supersonic and go sub-sonic as they slow down do have some weird behavior when they are transonic.
Yes- BC does play a role in wind drift.
But I'm having trouble with his “BC trumps flight time” rhetoric. BC may affect flight time but that’s different than what he seems to be saying.

 

[BBerguson]

I don't disagree. I think he wanted to write an article and found something that he could write about that probably nobody reading was aware of.

And the 200 yds as a chosen example confuses me. Can a modern air rifle shoot out to 200 yds and have any chance of consistency/accuracy?

Yes, the modern PCP rifles with slugs (bullets as we call them) will shoot along side a 22 rimfire to 200 yards and more. Watch this video:

 

[Petrol & Powder]

I don't doubt a modern air gun can push a projectile to 200 yards and do it with some hope of hitting a target.


I start having problems with air gun "slugs' (bullets or whatever term used) that have in combination of all the following: A. the BC claimed,
B. the weight necessary due to the shape and BC, and C. the initial velocity claimed.

In practice, you would want a projectile that had the best BC and start that projectile off below the speed of sound (but close to it) so that it never goes transonic (if it never exceeds the speed of sound, it never has to pass BACK down through the sound barrier).

When you toss is "air rifle" and kind of projectile weights involved, I get suspicious of velocities over 1100 fps.