Effect of Various Error Factors on Accuracy

[Oldblades]

Also momentum is less so it is easier to move it. So if there was a 5mph wind left to right for the first 50 yards then it switchs to a 5mph right to left your bullet would go left by quite a bit.

While the effect would be true at the point of occurrence, you have to remember that the initial effect alters the flight path that is then compounded by the distance.
 I dont know the math, but it takes a lot more correction to bring it back 'once the damage is done' out of the gate at those distances.
 
 

[Tomg]

Wind deflection has the most effect on POI the closer it happens to the muzzle because it alters the direction of the projectile while the distance to target is at its peak. As the projectile closes the gap to the target there is less time for the wind to alter its direction before impact.

[rsterne]

Correct.... Make the angles large and it will be easier to understand....

Start the pellet heading straight to the target, along the line of sight, and then before it gets to 10 yards add sufficient crosswind to change the course by 10 MOA to the left.... at 10 yards that is 1" left, 2" at 20, 5" at 50, and 10" left at 100.... Once you start the pellet out on it's new, angled course, if there is no further wind, it just keeps trucking along that new line.... constantly diverging from the line of sight by 1 MOA.... getting ever further to the left of where you aimed it....

Now, when the pellet gets to 80 yards, for the next 10 yards until it gets to 90 yards, apply exactly the same crosswind force, but to the right.... You have now straightened out the course of the bullet to PARALLEL to it's original course.... However, at 80 yards, it was already 8" to the left.... It now stays there, 8" to the left of your line of sight, until it hits something (assuming no more wind).... The same crosswind force, between the muzzle and 10 yards, changed the POI at 100 yards by 10" to the left.... compared to the equal but opposite force applied between 80 and 90 yards, which only changed it back by 2" to the right.... and the net effect was an impact 8" left....

This argument neglects the slightly decreased velocity at 80 yards, but depending on where you are on the drag curve, you may actually have a higher BC once the pellet slows down, and since the BC is the most important factor in wind drift, that same crosswind force may not even bring the pellet quite back to parallel to the line of sight.... Of course if the pellet is moving into a worse spot on the drag curve (it would have to be going really slow, <500-700 fps, for that to happen) the wind at 80 yards may have slightly more effect, but for the most part we can ignore these effects.... Wind near the muzzle is more important....

Bob

[Bwalton]

The only problem that I see when out shooting that a lot do not notice, but BR shooters do is that Over the course of 50-100yards, the wind can actually be blowing in several different directions at the same time! Infact you have wind guest to add in also. The best wind that you can get is one that is blowing steady in one direction, other then that you would need to line wind flags all the way to the target. Out in the field when hunting you do not have that option. Thats why before you hunt always check your zero, I find that my zero would change as it got hotter in the day.
Wind is also a good thing when it is steady and predictable. As long as it acts as a constant and not as a variable, we can calculate its effect on the flight of our pellets and make the necessary adjustments to compensate for it accordingly. It’s when the wind is unpredictable, gusting, and erratic that it plays havoc with us. More things come into play then just wind, like temp, and elevation. Poi can shift from morning to afternoon when things start to warm up. I try to make it a practice to sight in at every hunting spot if possible. I am trying to study the wind and its effects to the fullest, even understanding it  just a tad will increase you accuracy if you apply it....

[Oldblades]

  It’s when the wind is unpredictable, gusting, and erratic that it plays havoc with us. 

And how many times have you gone through the observations, then calculated holdoff, and the POI was exactly where you were aiming on your holdoff! 

Ribbonstone stated early in the thread that some days its better to just go fishing.

[Motorhead]

  It’s when the wind is unpredictable, gusting, and erratic that it plays havoc with us. 

And how many times have you gone through the observations, then calculated holdoff, and the POI was exactly where you were aiming on your holdoff! 

Ribbonstone stated early in the thread that some days its better to just go fishing.

It is a frequent giggle shooting Field Target ... shot after shot pellet go's where aimed, wind starts blowing and you spend the remaining time doping the wind and adjusting POA.
ONLY to be humbled with a few shots that fly exactly to point of aim WHICH WAS NOT ON TARGET having the wind which you adjusted for simply not there as pellets headed down range.

Yup ... adjusting for wind does have some science to it ... also a far bit of "Hail Mary" shots of luck as well.

[Rolling Thunder]

As has been pointed out - wind conditions outdoors are tricky, even with flags  .  Velocity and direction vary continuously, and are often different over the gun-target distance.

It would be interesting indoors to set up a high-powered fan to create a stable cross-wind of known velocity.  Then shoot (bench rested) with and without the fan, and measure average miss distance (POI center to X).

A wind probe could be used to measure wind speed.

One could also vary the fan speed, position, and angle to empirically test the effects on POI-POA, Magnus effect of different barrel/pellet combinations, etc.

[MicErs]

Wind doesn't always blow right left or front back, sometimes it blows up and down.  Shooting downhill into a head wind? That wind has a rising component.  Shooting uphill into a head wind?  That wind has a downward component.  Forget trying to put all this together in varying terrain with rocky outcrops.  Mirage causes an updraft when you shoot over it...  and so on...

[PakProtector]

It is interesting to note that .22 LR target ammunition (Eley Tenex 1055, one of the best in the world) has a ES of only about 25 fps over a box of 50 shells, which is 2.5%.... That translates into a vertical dispersion at 50 metres of 1mm per 3.9 fps, which means that over a box of 50, the vertical dispersion at 50 metres is 1/4".... Thinking that we need to have (or can achieve over 50 shots) an ES of less than 2% may be wishful thinking, mixed with theory and some VooDoo thrown in....

Bob

those rimfire cats doo all sorts of stuff even we'd call nuts in search of low-ES ammo...LOL Fortunately Lapua Center-X works well in the boy's 40x.
cheers,
Douglas

[jvanschoonhoven]

ok I am a bit confused now... I know for a fact that with my long range rifle the wind has a greater effect on the bullet the farther it is from my position. We are talking shooting 600 to 800 yard shots. Yes the MOA angle up close is carried all the way to the target so a 1 moa shift at 100 yards is 8 inchs at 800  but the truth is the bullet is so stable and going so fast when it leaves the barrel that it is much harder to move off course up close. The farther it gets the slower it is going the easier it is to move off course and the longer time it has to be moved. now the velocity drop from 20 yards to 800 yards is a huge ammount and maybe that is what makes this different with the pellet guns?

[rsterne]

Perhaps the area you are shooting in has much more wind downrange.... OR the bullet is so much higher off the ground, due to trajectory, that in the middle of it's flight the wind, which is much stronger further from the ground (wind gradient) is having more effect.... You don't notice it closer up because the highest point of the trajectory is nowhere near as far above the ground, trees, obstructions, etc. which decrease the windspeed due to drag with the wind itself....

If you study wind drift, you will find it is related to the drag coefficient, regardless of the type of projectile, and that drag makes a huge increase between Mach 0.8-1.2 (called the Transonic region).... and therefore so does the bullet/pellet drift.... It is the LOSS of velocity that is key to the amount of drift, ie the difference in flight time between a vacuum and the real world.... The velocity by itself is not much of a factor.... and in fact if the drag is higher at a higher velocity, you get a reverse effect, where increasing the velocity also increases the drift....

The drift, however, is a progressive thing, each increment building on the last, and if you use ChairGun, which has an excellent graphic representation of the drift vs. range, you will certainly see that the further you move downrange the greater the drift, and it is not even close to a linear relationship.... So, in terms of the pure amount of drift, there is a great increase between, say 400 and 500 yards, compared to 300 to 400, and even less at 200 to 300.... That, after all, is the thing you see, and the important effect.... As the range increases, so does the drift, and drastically....

Here is a good way to visualize what we are talking about.... Use ChairGun to plot the drift out to 500 yards (the maximum it can do).... Take a piece of paper, or a ruler, and set it tangent to the curve at any given point and see where it intersects the right hand axis, and compare that distance to the total drift....This is what would happen if you "shut off" the wind at that point.... The bullet would carry on that course all the way to 500 yards.... It will always be a greater proportion of the total drift than the distance you placed the tangent at is of the 500 yards....

Bob

[michaelthomas]

I think the difference may be in your view of what is happening.

If you were to head down to the indoor range in Grants Pass with a leaf blower.....you could empirically prove or disprove this idea in minutes.  Get your gun sighted in to shoot bulls.....then fire up the leaf blower 6 inches from the muzzle.  Note the POI shift.  Now do the same thing 6 inches from the target. 

The same force up close to the muzzle will shift the POI drastically, while the force 6 inches from the target will do very little.

Time in flight is not the sole governor of wind drift with a given projectile.....since the drag is not consistent at every velocity.  It is a very common belief that someone shooting the same pellet at a higher speed will have an wind advantage.....but it's actually not true in most instances.  I can't say for bullets.....but for pellets.....simply shooting pellet X faster will not necessarily result in less wind drift.  As an example....shooting a JSB 8.44 at 950 fps will give you no lateral wind advantage over shooting it at 800.  I have empirically proven this at my range with wind known wind strength indicators.  Not only does the higher velocity have no lateral advantage...... it produces greater vertical displacement via the Magnus effect due to faster spin, and it fouls the barrel a lot quicker.

Some of the above can be seen using a ballistic calculator like Chairgun, too. 

Mike

[michaelthomas]

I see Bob edited his post while I was writing mine.

[rsterne]

Good explanation, Michael.... Here is an example from ChairGun, using a muzzle velocity of 1000 fps and a BC of 0.10, over a range of 500 yards in a 10mph crosswind....

The drift at 500 yards is 84.88".... The drift at 100 yards is only 3.62".... So, if you look at the drift at 100 and multiply by 5, you only get 18.1", and so you say, "see, that proves my point".... Not quite.... The change in drift from 99 yards to 100 yards is 0.07", so the angle the bullet is travelling at 100 yards is 0.07" per yard.... Extrapolate that out another 400 yards, and you get 400 x 0.07 = 28".... Add that to the 3.62" it has already drifted, and you get 31.62", which is how far from your POA the bullet would land if you shut off the wind at 100 yards, and allowed the bullet to fly undisturbed all the way to 500 yards....

So, in the first 100 yards, that wind caused a deflection of 31.62 / 84.88 = 37% of the total.... However, 100 yards is only 20% of the distance to the target....

Likewise in the last 400 yards, the wind deflected the bullet an additional 84.88 - 31.62 = 53.26", which is 62% of the total drift.... but it took 80% of the distance to do that....

I hope that explanation, plus playing with a graph of wind drift and various tangents to it, will help you to understand why wind near the muzzle has more effect downrange than wind near the target....

Bob

[Oldblades]

I think the word 'stable' comes in to play also.
What round and load is being used, and atmospherical stuff can drag the speed down towards the transonic region.
 All kinds of bad things happen with that to a decelerating bullet at those ranges. Thus its just messing up a already altered flight path if that were to be in play.

[Ribbonstone]

When it really counts, you have to rush.  Might be hunting and you know the critter isn't going to hang around much longer or when you are up against a time limit in competition.  THEN you have to deal with whatever games the wind is playing right then.  So we look at the grass, look at the leaves, look at any wind flags,  try to calculate the various forces value...basically we guess.  Sometimes we guess right enough to not embarrass ourselves, but other times you have to wonder where that pellet got off to.

Better to stack the cards in your favor Take 5-10min. out at the start to just watch the wind. Get use to the ebb and flow; wind tends pick up/fall off/shift in a cycle. Hardly ever really falls off to nothing, but definitely seems to have repeating moments of predictability.  Would be better to shoot when you recognize the cycle. It's still a guess, but a better guess as you can learn something from previous shots under close to the same wind pattern. 

The 3rd type of wind guessing is to wait for however calm the wind gets.  This has always burned me, but I mention it anyway.  Wind is never calm, and trying this has only lead me into (1) rushed shots and (2) getting caught in a big wind shift (as it seems to calm down right before it decides to shift).

Some old benchresters gave me some advice that seems relevant.  Guys who wait for the perfect wind often shoot the single smallest group of the day...guys who recognize the wind's pattern are more likely to shoot the smallest aggregate score.

Considering my previous posts...keep some fishing tackle in the trunk so the day's not totally wasted.

[Sfttailrdr46]

  Yes some days it is just better to go fishing rather than throw the rifle in the river 

[jvanschoonhoven]

ok so I have been playing around with chairgun and have run a few tests please correct me if I am wrong here. shooting a jsb .25 cal pellet with a bc of 0.0360 shooting at 950fps with a 10mph crosswind it has a 5.2 MOA drift at 50 yards and is going 796fps at 50 yards. lets assume we are shooting 100 yards..  If I put in 796fps as a starting velcity and do another 50 yards the drift is 5.1 MOA.  So in this case the closer wind effects the pellet more. now lets push it out to 200 yards and try it again. 

This time in the first 100 yards there is 10.3 MOA of drift and the pellet is traveling at 685fps
Starting over with 685fps as our starting velocity the drift is 11.9 MOA so in this case the wind farther out effects the pellet more. 

I see now why the long range shooters I know say the wind farther out effect the bullet more becasue of how quickly a 308 bullet slows down over 1000 yards the velocity at 1000 has dropped so much that the effect of the wind is greater. however with pellet guns its a bit more tricky.
I also see if the wind is blowing in a single direction then what you are saying is defanantly correct however if you have changing wind directions over long distance then it is different. To clarify my earlier post I was talking about wind chaging directions not just a 5mph wind right to left up close vs at the target. I see now that I did not make that clear at all.

Let me know if any of this makes sense.

[rsterne]

You need to read the post I made about using a tangent to the curve.... While in your first example where the pellet has drifted 5.2 MOA at 50 yards, look at the ANGLE it is on at that point (ie where it is pointing).... You can't just draw a straight line from the muzzle through 5.2 MOA and extrapolate it out to 100 yards and get the same 5.2 MOA.... The pellet, at the 50 yard point, is already pointing much further to the side at 100 yards than 5.2 MOA.... Even if the wind quit at the 50 yard point, it would land much further than 5.2 MOA from the POA.... I can't get my ChairGun to spit out your numbers, to give you an example, what Drag Profile are you using?....

If you look at each 50 yard increment, and assuming the BC stays constant over the entire velocity range (it doesn't), then as the velocity drops, you are correct, the bullet is affected more by the wind IN THAT 50 yd. INCREMENT.... However, let me give you an example.... Let's say the wind blew left to right for the first 50 yards, and then reversed and blew right to left for the next 50 yards, at the same strength.... The bullet will land to the right of the POA at 100 yards.... because the initial deflection in the first 50 yards is amplified when you double the distance to the target....

Bob

[jvanschoonhoven]

So if I understand what you are  saying correctly because of the angle the pellet is traveling at the 50 yard mark even though the pellet is .pushed back farther in the next 50 yards it still lands to whatever side the first wind was blowing?