Okay, first: Is “precise estimates” an oxymoron? Well, perhaps, but if precise isn’t absolutely true, how about more precise?

As I mentioned in my recent thread about shooting at up or down angles, most shooters are familiar with certain things that affect our ability to hit a target, but very often that familiarity is very limited, and often based more on misinformed myth than sound facts. This post is about one such factor: the effect of wind on bullets’ flight and what the shooter can do about that. But before going further, and despite its length, this thread won’t turn anyone into a wind master; the subject is very complex, but hopefully this will be a start for those interested in the subject.

Professional snipers and other highly experienced shooters like precision rifle competitors who shoot a couple of hundred rounds a dozen times a year can make informed judgments based on their training and experience, and this thread isn’t for them (although even they may get something out of it).

The simplest part of wind and ballistics is that if we know, for example, that the wind will move our bullet a foot off course by the time it gets to the target, then all we must do is aim a foot in the other direction so the bullet is blown onto the target rather than off. If we know the speed of the wind that’s affecting our bullet and we have a good ballistics solver, it will tell us how much aiming correction must be made.

But what if we don’t have a solver at hand along with a wind speed meter to tell us how fast the wind is blowing?

Where I usually shoot the wind is highly variable; it changes direction and speed literally in the space of a few seconds. Even though I have a good wind meter, there’s no way I can use it to give me accurate holds when I’m getting set up for a timed shot. I must make quick estimates of the effect of the wind on the bullet I will be shooting, adjust my aim accordingly, and make the shot in the brief window of that particular wind speed and direction.

The method I’ve developed to help me do that is what I call the Standard Wind Unit (SWU), and as a disclaimer for those who may already be familiar with the concept, I conceived and developed the concept for myself long before I learned that I’m not the only one who figured it out. My methods are more precise than one I’m familiar with and, I believe, easier to understand and use than another.

After all that introduction, a brief description of how using the SWU works. For our example, I’m using the 308 Winchester 168 grain Gold Medal Match load at 2600 feet per second muzzle velocity under standard atmospheric conditions.

For that load and conditions, the Standard Wind Unit value is 5 miles per hour when using a scope reticle calibrated in milliradians (mils). With a full value wind that’s blowing from the side at 3:00 or 9:00 o’clock, a 5 mph wind will move the bullet about 0.1 mil per 100 yards of target distance. That is, for precise aiming in a 5 mph full crosswind, we need to aim into the wind 0.1 ₥ for a 100 yard target. But what about different wind speeds and directions? The good thing about this method is that the effect of the wind and therefore our aiming adjustments are linear to several hundred yards.

This table shows the effects of a 5 mph full value crosswind in mils at the distances shown for our example load, and that’s the basis for determining what our SWU value will be:

100 y: 0.09 ₥
200 y: 0.19 ₥
300 y: 0.30 ₥
400 y: 0.41 ₥
500 y: 0.53 ₥
600 y: 0.66 ₥
700 y: 0.80 ₥
800 y: 0.95 ₥

Note two things about the chart.

First is that when the milliradian values are rounded to the nearest 0.1 ₥, the distance in 100 yards equals the windage effect to 500 yards, and it’s pretty close even at 600 yards. 100 = 0.1 ₥, 200 = 0.2 ₥, 300 = 0.3 ₥, etc. In other words if we know that the wind is blowing from 3:00 o’clock at 5 mph and the target is at 300 yards, we hold off 0.3 ₥ to compensate for the wind.

The second thing is that as the bullet velocity drops at the longer ranges, the linear change doesn’t hold true, and it becomes necessary to add to the degree of wind hold aiming adjustment. But in our example, it works well to 500 yards.
I used a ballistic calculator to determine the Standard Wind Unit value for the example load. After entering all the other ballistics data, I entered different wind speeds and with the direction of the wind being 90 degrees or 3:00 o’clock from the line of fire. For each wind speed I checked the windage value for the various distances out to 500 yards. When the windage value for the wind speed entered corresponded to the distance, e.g., 300 yards = 0.3 milliradians, that was my SWU value, and in this case 5 mph.

Another common SWU value for sea level conditions is 4 mph, and that’s a good place to start. At higher elevations, the SWU value will increase. At my altitude the SWU value for one 6.5 Creedmoor load is 7 mph because of the bullet’s higher ballistic coefficient and the thinner air that has less wind effect on the bullet. That means a full value wind must be blowing 7 mph to move the bullet 0.1 milliradian at 100 yards.

What about different wind speeds? Again, the effects of the wind are linear at different speeds. Using the same ballistics as the original example, these are the milliradian values for different speeds at 300 yards.

10 mph: 0.61 (0.6) ₥
15 mph: 0.89 (0.9) ₥
20 mph: 1.18 (1.2) ₥

In each case when the value is rounded, the result is just 2, 3, or 4 times the original 5 mph value at 300 yards of 0.30 ₥.

The next thing to consider with using the SWU method of estimating wind holds is the direction of the wind.

The below diagram may be a little hard to read, but it is available elsewhere on the Internet. Be careful, though, to pick one that shows accurate values. A common error is to claim that a wind coming from 1:30 on the clock (45 degrees) is a “half” (1/2, 0.5) value wind, and that’s simply wrong. The effect of a wind from 1:30 o’clock is 3/4 (0.75) of a full value wind. A 5 mph wind from 1:30 has the same effect as a 3.75 mph (round to 4) wind from 3:00 o’clock. A 1:00 o’clock wind has 1/2 the value of one from 3:00 o’clock.



The wind direction must be factored in because, as indicated, that affects its effect on the bullet.

So, how does all this get used? It’s a simple mathematical process.
1. What is our Standard Wind Value? Assume we’re using my example load, and therefore it’s 5 mph for each 0.1 ₥ adjustment per 100 yards.
2. What is the range to the target? If it’s 300 yards, then use (3).
3. What is the wind speed? For this, it’s 10 mph, or two times the SWU value = (2)
4. What’s the direction of the wind? It’s from 1:00 o’clock or 30 degrees = (0.5)
5. Convert to milliradians (0.1)

To calculate our hold:
3 × 2 × 0.5 × 0.1 = 0.3 ₥ hold into the wind.

If it were a full value 15 mph (5 × 3) wind from 3:00 or 9:00 o’clock and a 300 yard target:
3 × 3 × 1 × 0.1 = 0.9 ₥ hold into the wind.

By this time anyone who has made it this far may be thinking that the arithmetic is simple enough, but what about the difficult part: determining the wind speed? That’s really a separate subject to consider and books have been written about it. The most accurate way of determining wind speed at the shooter’s location is with something like a Kestrel weather monitor, but if we have such a device, why not have one with a ballistics solver that will automatically calculate the wind hold necessary? That takes me back to my original statement about how certain situations and conditions can make using such a device impracticable, and the need to rely on estimates.

Because of the occasional need for estimates, I followed the advice of other long range shooting authorities to develop the ability to estimate wind speed. Outdoors I use my Kestrel to read the actual wind speed, and then note what effects different speeds had on me and the local environment. It didn’t take too long to get to the point that I could determine wind speeds up to about 20 mph pretty accurately. As I say, though, no matter how we do it, having a good estimate of the wind speed is fundamental to any method for adjusting for the wind.

As a last comment, the whole process can also be used with scope reticles calibrated in minutes of angle. Only the specific mathematics are different, and for example we might come up with a Standard Wind Unit value of 12 miles per hour per minute of angle at 100 yards. In that case, a 12 mph wind would move the bullet 2 MOA at 200 yards, and so on.

Rifles Only touches on similar methods to estimate wind holds in their initial precision rifle courses. Many pre-printed rifle dope books also include methods along this line. A few students asked our instructors about the need for this, especially given the ballistics programs available. Instructors almost always state that the methods work if your pre-printed dope cards fly away or if your electronic ballistics solvers go tits up.

Being the jerk in the back of the room, I asked if the instructors if they every used such methods. << crickets >> Then "nope". Same thing from the SWAT, designated marksmen, and snipers I've talked to. They all used some form of calculated ballistics dope, until they shot enough that they had the data essentially memorized.

When one shoots a caliber with a given load long enough, one develops a pretty good handle for wind holds.

I have dope cards. I may refer to them once in the morning, or for every stage. But I use dope cards.

I will confess that I did not read the OP in its entirety. As I got further into it, my growing thought was "who has time for doing this before a shot?"

My next thought was "this is using a supercomputer to calculate ultra precise values from questionable data, then measuring precisely with a laser to the fraction of a millimeter and then cutting with an ax."

I apologize for not arguing the theory and the algorithms behind all this; I will just talk pratical application. Well, fritz took away a lot of my thunder with his superb on-point response. I'll just cover it from the F-Class POV. This is the discipline in which we shoot at well-known distances on exactly the same target at distances from 300 yards to 1000 yards. (The circles on the target are all the same "MOA measurements" for all distances.) We have flags on the range that are supposedly standard, meaning they should have the same deflection amount for the same wind value, anywhere you go. (Should is a big word here.)

In 17 years of shooting F-Class all over North America, I can count on the fingers of one hand how many times I have seen windmeters being used. There was a time when everyone had one, yes, even I did, but they rarely came out of the bag once the shooting started and then they simply stopped being taken out of the bag, by about 2008. Now the only times I have seen one in this decade, it was wielded by a new shooter. I am aware of the new gadget that you place far downrange, and which measures the wind velocity and direction at that distance and transmits the info back to your device. Totally illegal in Highpower competition, but great training aid. I would think it would be frowned upon in PRS competitions.

The windmeters only measure the wind at your location. It doesn't know anything about the wind at 500, 800 or 1000 yards downrange. When I sit on the line and I watch the flags on both sides of the range and they are all pointing to each other, or the ones between the line and 600 yards are limp and the ones at 800 are standing straight out, I don't think a windmeter at the line would help me much.

As I wait for the match to begin, I look at the conditions and try to determine the predominant condition. The wind changes all the time, but it usually has a predominant disposition, shall we say. If I see that the wind is generally blowing from the right and it appears to be about a certain velocity, in my mind, that translates to a general MOA setting. Let's say the flags indicate about a 2 MOA breeze, I'll dial 1.5MOA right and then stay aware of the conditions. Of course, I could be poaching on other competitors to see how they are doing on the target; that's part of the game. But let's say you are the first one, or the only one to shoot. I know my load; I know my bullet and I know how much it moves depending on how the flag unfurls. There are still a number of things that are tricky. The direction of the wind, which can be very hard to determine from the ground hundreds of yards away, and how quickly the conditions will change.

To monitor the distant flags more precisely, I use a spotting scope when in position. I am now using an ultra-wide eyepiece in my Kowa that allows me to see a great deal of the target line along with most of the flags on one side or the other. I want to see the target line because I want to see the mirage flow.

The mirage is critically important and that will help refine the hold. It is also my indicator for upcoming condition changes. A most difficult thing however, is to guage the velocity of the mirage and its actual direction. Then you need to translate that into what it will do to your bullet.

So, I will make an educated guess with what I am seeing and hold on the ring that I think I need to use as my aiming point and press the trigger. As sson as my followthrough is done, I look in the riflescope to see if the conditions have changed. Then I reload and wait for the score to show up. I like it when the first shot is a 10 or an X. That's very satisfying, but I'm happy with a 9 as sighter 1. If I get worse than that, I know I screwed up somewhere, usually by a condition that I either under or over estimated, but sometimes by a condition that I did not see, and that's worrisome. Again, this would be something that you would not detect with your little windmeter at the line. The flags could be wet, or they could be made from a different material, or I just plain screwed up. During the match, I try to be as alert as possible to the condition changes and mirage is usually my first indicator, but sometime a specific flag, or set of flags speak volumes. Or they can lie.

I recently changed bullets and adjusted my handloading regimen. Last month was my first outing with that load and I struggled a somewhat as I was overcorrecting. I did discover that my waterline was under 1 MOA at the 1000 yard target all day long (did I just say "under 1 MOA all day long?) all my lost points were to wind, left and right. This past weekend, the waterline stayed under 1 MOA, and I was much more comfortable with the bullet. The result was a high master score for the day with 30% X-ring in switchy conditions. Good enough for second place.

You can try to calculate your way to better accuracy, but ultimately it's going to be experience that does it, experience with the load and the rifle. Once the match has started, you monitor the conditions, observe where your last shot went and automagically make the adjustment, if any, from there while watching the conditions. Letoffs are a b*tch; I've lost more points to letoffs than I care to think about. Know when to hold fire.

The problem with letoffs is that in pair-fire, you do not have the luxury of waiting, you have to take the shot and that's when you must know your bullet well and be able to recognize and adjust accordingly to the conditions and take the shot. You do not have the time to go back and redo your calculations.

There simply is no subsititute for experience and shooting the same rifle/load in various conditions. You don't stay home from a match because it's too windy. Everyone will be affected by the wind, and you will learn a great deal.

Please, NikonUser, What are 'letoffs' and 'pair-fire'?

My apologies. When I get on a roll, I tend to use vernacular for efficiency, forgetting that some of it can be very obscure.

Letoffs is when the wind suddenly dies down or slows down considerably. You're shooting 10s and Xs while holding 4 to 5 rings left on the target. You're pumping in rounds as quickly as you can. Then all of a sudden your score comes up an 8 at 9 o'clock. Almost exactly where you were holding. The wind coming from the left simply stopped, a big letoff. You realize that when you take a second or two to look at the flags, something you neglected to do in your frenzy.

Pair fire is a competition format in Highpower, especially in ICFRA, what used to be Palma. You are paired with another shooter on the same target. You will be shooting together on the same target, one at a time. You exchange scorecards and get ready. The first shooter takes a shot. The target is scored. The other shooter calls out the score. The first shooter accepts the score by saying Thank you. The second shooter records the score on the first shooter's scorecard. The second shooter now has 45 seconds to take his shot. The first shooter will call out the score and record it on the second shooter's scorecard when he hears Thank you. Then he has 45 seconds to take his second shot, and on it goes. You get to wait maybe up to a minute between shots and you have 45 seconds.

This is also known as two-to-a-mound. It can also be 3 to a mound. That gers even trickier. This means it can take up to 2 minutes before your next shot and you only have 45 seconds. This is not like string fire where you have 30 minutes to shoot your 20 rounds for record and your sighters. You can shoot as fast as about every 7 to 10 seconds, or stop and wait as long as you want within the 30 minutes. I have seen people rip off 20 rounds for record in about 4 minutes. I have seen people take the whole 30 minutes and every combination in between.