Wind Uprange And Downrange

[ErikW]

I was watching an episode of Mail Call wherein R. Lee Ermey visisted the scout-sniper school of Uncle Sam's Misguided Children. The instructor said, and I'm paraphrasing, "Don't worry about the wind at the shooting position, worry about the wind downrange at the target."

Huh? I was told and/or read that wind nearer the muzzle has more affect on bullet path than wind nearer the target.

Intuitively to me, it would seem the downrange wind would have more effect, as the bullet is moving slower and is pushed by the wind for a longer duration. But a former Highpower shooter told me this was not the case, that wind closer to the shooter had much more effect.

[blkbrd]

Wind will have more effect closer to the muzzle, use any of the free internet balistics programs and you can calculate it out. Say a 5mph wind out to 800 yd. as oposed to a 15 mph wind from 600 to 800.

Even though the projectile is moving faster if you have a small deflection close in it will affect you more than a decent size one at longer range.

Try sniper.com, or varment.com or some other precision shooting sights for links to balistics programs.

[Flexmoney]

Marine trained snipers use majic bullets that are not effected by the normal laws of phyics.

...and don't get me started on whether we really went to the moon or not.

if you have a small deflection close in it will affect you more than a decent size one at longer range.

That makes sense.

[George]

This is a tricky one because in a way that Jarhead is absolutely right if you are comparing only the wind across a given distance at the higher velocity present from the muzzle to a specified distance versus the reduced velocity of that same round starting at 300 yards and then for the next 300 yards on out to 600.

Here is what I get from my ballistics program with the starting parameters as follows:

.223 caliber, 55gr, flat base, pointed nose projectile. BC corrected for G6 drag model = .123

Here are the results:

3100 fps muzzle velocity in 10 mph wind gives 13.5" deflection after 300 yards travel

1910 fps muzzle velocity (this is what the 3100 fps muzzle velocity has decayed to by 300 yards) in 10 mph wind gives 32.5" deflection after 300 yards travel.

The velocity is what determines the wind bucking potential of a projectile, and as that decays, so does the wind it will take.

Wind is not always constant across the entire flight path, especially at the real long distances that snipers have to negotiate. I have shot IPSC rifle in conditions that had esentially no wind across the first 150 yards, then 20mph gusts in the next 100 yards, then no wind for the last 50 yards and I have shot Highpower where it was gusting enough to push you around at the line, but the flags were hanging limp at the targets. The real world is never so generous as to present a constant variable to us ;-)

Regards,

[Guest Larry Cazes]

Yup....I have to agree that the real answer as is often the case........IT DEPENDS. Distance, Velocity, BC of the bullet all contribute to deflection.

[EricW]

I was told and/or read that wind nearer the muzzle has more affect on bullet path than wind nearer the target.

I'd better just move to Missouri now. Somebody had better show me.

I've done a buttload of wind compensation calculations as a pilot and as far as I know, the wind is the wind and the vector sum of the crosswind component plus the bullet's velocity should yield the trajectory. If something magical happens when a bullet goes supersonic then somebody needs to 'splain.

Yeah, there's some extra complications like the non-constant speed of the bullet, intertia, and the fact that the bullet takes some time to accelerate to match the crosswind component, but my BS detector is in alarm mode on this one. Simply looking at the triangles resulting from a constant crosswind coupled with the bullet velocity at the muzzle and downrange would indicate that trajectory is more affected downrange than uprange.

[Please nobody argue from a 3rd party authority on this one. Either you know the answer or you don't.]

[ErikW]

I retrieved John Plaster's The Ultimate Sniper from the bookshelf; it doesn't have a lot to say on the subject.

... the reason you prefer shooting through a farther wind is that there's less remaining flight time to be affected by the wind.

... you should time your shot so one wind is calm and the bullet passes through the other ... the near wind has more time to push it sideways and will cause it to be much farther off target than a far wind. Therefore, you should shoot when the near wind is calm and compensate for the far wind.

He also has an illustration that shows the near wind pushing the bullet farther off-target than the far wind.

[George]

He also has an illustration that shows the near wind pushing the bullet farther off-target than the far wind

Near wind, far wind, any wind at all (my friend Buff would have great fun with that one!), you have to ignore velocity drop for that to be true. And it's Not!

If the velocity drops, the trajectory deflects more in a given wind (vector decay) Period. If you don't have an engine to add energy, then the velocity will drop at a specific and ever increasing rate that the projectile shape helps determine. The big BC VLD's work as well as they do partly because they don't shed velocity as quickly over the flight time (better drag coefficient).

The slower it goes, the more it drifts. Read whatever wind you want, correct for whatever wind you can. It's all in the speed. Keep speed constant, and the deflection will be constant. Let speed drop, and the deflection will increase. Can anyone say "time to target" ?

Note the results I posted earlier. Remove 35% of the speed and watch the deflection go up by more than 50%. Nuff said.

Regards,

[George]

BTW, Eric, your BS detector went off for good reason.

David Tubb doesn't shoot as well in the wind as he does because of "which" wind he always shoots in. It's because he always chooses the "right" wind to shoot in, no matter which one it is.

Regards,

[EricW]

... the reason you prefer shooting through a farther wind is that there's less remaining flight time to be affected by the wind.

He also has an illustration that shows the near wind pushing the bullet farther off-target than the far wind.

I'll have to pull out my copy of ahem, the "Ultimate" Sniper, but I still say it's a crock. How can that even be possible? The bullet is travelling SLOWER downrange, therefore the flight time is LONGER leaving more time for it to be affected by wind.

I'm going to have lay siege to all of MO until someone *SHOWS* me how this theory could possibly be true.

[George]

Here's the numbers, and they don't lie. This whole thing may, or may not be a "total" crock, but the chart flat out shows that you are better off shooting through a 10mph crosswind that is present only in the first 200 yards, than you are in one that is only present in the last 200 yards across 600 yards.

0-200 = 5.3" deflection 400-600 = 50" deflection. If you only get about 5" in the first 200 yards, and then the wind goes away for the next 400, that initial 5" translates to an additional 15" (or 2.5 MOA) of deflection by continuing on the same course. This is a no brainer.

Regards,

[EricW]

So, it looks like me, George, and R. Lee Ermey versus John Plaster and the entire benchrest (and possibly 3-gun) community.

Let the games begin.

[George]

Let the games begin

I'm in!

Once I'm certain (whether I'm right, or I'm wrong), I'm willing to stand my position until the last.

Lead on, Leonidas

[Flexmoney]

I think all that is being said is that if you are blown off course early...you continue down the wrong path. The sooner you are of course...the more off course you will be at the end of the journey.

[George]

I think all that is being said is that if you are blown off course early...you continue down the wrong path. The sooner you are of course...the more off course you will be at the end of the journe

Blkbrd's example of 5 mph across 800 yards versus 15 mph from 600-800 yards is not exactly fitting the "logic" of a near wind versus a far wind. 5 mph across 800 yards is not a near wind, it is a constant wind across 3 times the distance of the compared to segment and it is cumulative, ie; once the error is induced the angle diverges and keeps doing so on a new vector that is compounded by additional side force (read wind). This is not, I repeat NOT a fair comparison like the velocity compensated examples I have given are.

Once again, I stand my position! An equal wind across a specific distance against a downrange (read velocity dropped) projectile will incurr more drift every time versus an uprange projectile (read velocity not dropped) across the same distance. Start mixing parameters unequally and we have no valid comparison to be made.

To be a valid comparison, it's got to be apples to apples, not apples to watermelons.

BTW, we are talking vector decomposition and orbital decay type dynamics here, not basic linear geometry as played out in whacking billiard balls in a series of intersecting straight lines.

Regards,

[rhino]

How much of the increased drift at lower velocities is due to the bullet being easier to deflect when it's moving more slowly, and how much because it's exposed to the force (wind) for a longer period of time for a given change in downrange distance?

[George]

It very hard to peg that as an exact constant. Because the BC or drag coefficient drops slightly in stages as the velocity sloughs off kind of linearly.

BTW, you cannot just compare different sections of an overall chart because the downrange deflection is compounded by the earlier deflection so a separate run for each variable change must be made to see anything meaningful.

With all of the same starting input data I have been using (see my first post), here is one set of comparisons.

Time to target with starting velocity @ 3100fps for a 200 yard distance is .224 sec for 5.38" deflection in a 10mph wind. Velocity is down to 2296 at 200 yards which is a 35% drop.

Re-run the table for a starting velocity of 1556 fps (the velocity it would be doing at 400 yards) and here are the results. Across 200 yards in the same constant 10 mph wind, time to target is .482 seconds for 16.88" deflection and you get a velocity drop to 1043, or 49%.

In the second example the velocity drop incurred is 1.14 times greater. The time to target is 2.15 times greater. And the wind deflection is 3.14 times greater.

I'm not saying I see a pattern here yet because this would need to be run for many, many differing projectile and velocity combinations before any valid conclusions could be drawn, but it does seem to have a certain linearity to the relationship of time & drag so far.

Regards,

[DogmaDog]

I'm not a Marine sniper, but I was a member of the Marine Rifle Team. The Marines do indeed teach shooters to look at the wind downrange to make their wind calls, for the reasons George has explained. Where the bullet is travelling slower, it spends more time, so it gets pushed further by the wind.

Rhino: the effect is soley due to time spent--there is no change in the ease with which the bullet can be pushed (shoot a bullet out of a horizontal barrel, and drop one from the same height at the same time, they hit the ground simultaneously). I think of the bullet as a boat going across a river...it's velocity downstream equals the current velocity, regardless of how fast it is going cross-stream.

That said, calling the wind is an art, and the better Marine marksmen use all available wind data to make their calls. If the flags in the pits are limp, but the flags behind you are blowing, you need to slap on some windage, but it's less than if the flags in the pits were blowing, and behind you were limp. We'll also throw some grass or leaves in the air on the line, to see how they blow, and look for mirage in our spotting scopes (mirage is probably the best indicator, as you see the wind along the whole trajectory).

Look everywhere, do an on-the-fly integration of all the indicators, and make a call.

DogmaDog

[George]

the effect is soley due to time spent--there is no change in the ease with which the bullet can be pushed

That is absolutely correct when referring to bullet drop in a fixed gravity. Time to target is all that matters in regards to gravity which is acting on the projectile with a fixed force which is independent of projectile velocity.

If time to target were the only deciding factor in wind deflection, the 2.15 multiplier for time to target in my second example would give exactly 2.15x the wind deflection instead of the 3.14x you will actually get (with that BC projectile, at that velocity anyway).

The additional factor at work here is the ever-increasing drag the projectile exhibits as the velocity sloughs off. In bullet drop the projectiles changing drag coefficient only contributes to more time spent in gravity. In deflection, not only is more time spent, but the side force applied by the wind is increased as the drag resistance of the projectile decreases along with the velocity (BTW, this is not a linear relationship). In the example I calculated, this ratio is 3.15:2.14 or 1.46 to 1.

Regards,

[Mikelly]

Often times, when people refer to reading "near winds" versus "far winds" they are experiencing conflicting wind patterns (which happen with some frequency on large ranges and valleys where the wind will "swirl"). In these cases, doping for the far winds is preferable as they are the winds that will have the greatest effect on downrange impact. I have shot through 35 mph gusts at 100 and 200 yards with little shift in point of impact. I have shot through 12 mph winds that put me in the 9 ring at 600 yards. An associate of mine shot through jet engine blast at 70 yards with negligable shift in point of impact.

[George]

I have shot through 35 mph gusts at 100 and 200 yards with little shift in point of impact. I have shot through 12 mph winds that put me in the 9 ring at 600 yards. An associate of mine shot through jet engine blast at 70 yards with negligable shift in point of impact.

Absolutely, I have noted that too. At out to 150-200 yards I have never worried much about .223 drift with 55gr, 69gr or 77gr loads in IPSC rifle shooting or in High Power. Winds near the target, or in the middle of a long flight path can indeed have a very large effect once the velocity drops off and the projectile spends any time in them.

At the 2002 3gun nationals, the LD rifle stage had 2 MGM flashers at about 325 yards. The shooting position was in a narrower section of the bay and was sheltered by the side walls for about the first 125 to 150 yards. It was relatively calm in this area most of the time. I took the opportunity to walk out to the flashers and back at lunchtime before our squad came up. I noticed that as soon as I passed the lee of the midpoint and the walls of the bay receded I was hit by gusts of wind that kept changing direction and battering me side to side on irregular intervals of 3 to 8 seconds with similar periods of calm until I got right to the targets where there was a little shelter from the wind (this made misses into the backstop blow dust with a false value if you were trying to read that).

After the match I ran the numbers on the 55gr boat tail I was shooting at 3100 fps and found that after the velocity dropped to 2500ish at 150 yards out, that it would experience 7-9 inches of deflection in a 20mph gust across the last 150-175 yards.

I am not absolutely certain that it wasn't the tiniest bit of imprecision on my part, or if I really did get clobbered by this, but I did hit flasher 1 clean on the first shot then missed flasher 2 with two identical sight pictures in a row (scope at 5x), and then hit it 3rd shot with the exact same sight picture all the while watching my misses into the backstop look like they were just barely off the plate and not showing much of any dust blowing. Turned a really good run into a just OK run (17th). After I reasoned this out later, I switched over to 69 & 77 grain projectiles for 3gunnin' in the wind.

Regards,

[BPiatt]

You guys are making this a lot harder than what it actually is....

Think of this example....

Forget about wind for a minute and think of this....

What if you were able to push/point your bullet to the side 1/4" as soon as it left the barrel. How much would that bullet be off at say 500 yards..... I'm not getting out my computer to do the math but I can tell you it will be a LOT.

Now, push the bullet that same amount at 400 yards. How much would it be off at 500 yards?

Make sense?

Bruce

[lmccrock]

push/point your bullet to the side 1/4"

Bruce,

Ok, push or point? If a bullet is pushed sideways by 1/4", then at 500 yards, it will be 1/4" off due to that push. If a bullet is pointed 1/4" off, then it is now aimed way off.

So does wind change where the bullet is pointed, or does it just push sideways? That is, does wind which exists for a short portion of travel blow a bullet off course onto a new vector, or does it just push the bullet sideways?

I suspect that the wind pushes the bullet sideways; if the wind only exists for part of the trajectory, the bullet is still pointed in parallel with the barrel. I have no evidence other than looking at vectors; I am sure someone will chime in.

Lee