Wind Drift; First Third vs. Last Third of Target Range

[Bart B]

This came up in a conversation about which crosswinds effect bullet drift the most; those closest to the firing point or those closest to the target. Here's Sierra Bullets' software plot for their 30 caliber 180-gr HPMK bullet leaving at 2650 fps. Green horizontal bullet path's that for a 10 mph crosswind from zero to 333 yards, red one's for bullet path with the same crosswind from 667 to 1000 yards:

Wind in first third, about 44 inches at 1000 yards.

Wind in last third, about 16 inches at 1000 yards.

 

[Len Backus]

Bart, thanks so much. Very striking demonstration.

 

[BryanLitz]

Academically, this is accurate.

Don't mistake the information to mean that the wind near you is always more important to pay attention to than the downrange wind; it all matters.

In the real world, there are many scenarios where the wind is most decisive over some segment of the bullets path, and it's not always close to the shooter.

I'm not arguing the information or the point, just adding context for consideration.

-Bryan

 

[Bart B]

Here's another interesting thing about wind. The higher a bullet is above the line of sight, the greater the cross wind speeds will be. Consider this table I built using the formulas that helps people installing wind turbines for generating electricity:

A .308 Win bullet will have a maximum ordinate of about 3 feet above the line of sight on its way to a 600 yard target. Same cartridge zeroed at 1000 yards, it's bullet goes about 11 feet above the line of sight to the target. For a .300 Win Mag, the maximum ordinates are about 2/3rds as much. Both maximum ordinates are about 55% of the zero range.

If the cross wind's 15 mph fifteen feet above the line of sight across all the terrain types shown across their columns, the actual wind speed in the line of sight is shown in bold text.

All of this is why the first 20 or so firing points at the west side of the 1000-yard range at Camp Perry were eventually not used. The tree line on the west side was so close to the first 20 or so shooters, they had a wind advantage of getting less cross winds from the west that those at the east side of the firing line some 70 firing points further from the tree line.

As far as I know, none of this is used in software calculating wind drift for bullets. It all assumes the same crosswind speed for all bullet heights above the line of sight.

I can imagine what the cross wind speed is some 47 feet above the line of sight 45 caliber 500 grain lead bullets had shooting 1000 yard matches in the late 1800's.

 

[ShtrRdy]

Good info. When I first started to read the thread I was thinking, "now what did Bryan Litz write in his book on this topic?" I recalled it wasn't a simple answer but you captured a basic example on the topic. Thanks -- Todd

 

[Mikecr]

Bart, do you know if Sierra has tracked drift with RADAR?
I'm not concerned about their solution, but of the track itself.

 

[Bart B]

Mike, I don't know if Sierra has or not.

Some years ago, I talked with their ballistics manager, Martin Hull and either Bob Hayden or Bill McDonald about spin drift. In so many words, they said it's too small and insignificant to put in any ballistic tables. It only comes into play at the longer ranges and is too often masked by subtle cross winds. If corrected for a mean windage zero with a .30-06 shooting a 180-gr. HPMK bullet through 1000 yards, a 1/3 MOA error at most would happen. A little to the left through about 700 yards then about the same amount to the right at 1000. Total drift's about 7 to 8 inches. Easily masked by a subtle cross wind switching back and forth from 1/2 to 1 mph. Nobody I know of can consistently measure all the cross winds' drift in a bullet's trajectory through 1000 yards to correct for within a 1 MOA error.

I've tried to use JBM's drift calculator but cannot see spin drift listed in any output column. Used a Sierra 30 caliber 180-gr. HPMK leaving at 2700 fps zeroed at 1000 yards. Unless it's the windage column stating about 7.6 inches.

 

[Mikecr]

I'm talking about wind drift.

 

[Bart B]

I'm talking about wind drift.

Well then, I erred a bit. I don't know if Sierra nor any bullet maker tracked wind drift for sporting rifles. Some military projectiles have been tracked by radar to get their muzzle velocity for use in computers calculating where to aim them.

I made my own formulas based on the old military one (range in hundreds times wind speed divided by 10 (a constant) equals full wind correction value in MOA for winds at 9 and 3, use half the MOA value for winds from 1, 2, 4, 5, 7, 8, 10 and 11 o'clock). Used Sierra's software to get my own constants for each range as the military one only got you on the 6-foot square target paper through 1000 yards. My own constants were pretty accurate; within a couple MOA anyway based on actual wind speeds measured at different ranges.

 

[Mikecr]

Well I don't even know if this should be called 'drift' or 'deflection'.
In my mind, aerodynamic deviations(active) to a bullet's fall should be referred to as deflection. Those deviations that are not taking energy from the bullet's condition(passive) should be referred to as drift. That's just my thinking.
There was an incredible discussion on wind deviations at BRC that went roundy roundy for a long time, ultimately with zero conclusions about it.
One camp thinks a bullet points into wind and gets sucked sideways like a wing. These folks think bullets fly..
Another camp thinks bullets simply point toward a new path of least resistance. These folks think bullets fall.
I think it may be both, in that a bullet will point forward even with a drift as this takes least energy(for a gyroscope), and point forward into relative vector amounts to least resistance aerodynamically. But I believe the actual deviation is still passive, for falling bullets.
Fire control RADAR could show it.

The formula for wind deviation is based wind component applied to time lag:
D=W(T-R/V) where
D = bullet deflection in feet,
W = crosswind in fps,
T= time of flight in seconds,
R/V = range in feet divided by muzzle velocity in fps.

This means no lag = no deviation. So,, if you drop a bullet straight down from 100yds up, into a 10mph crosswind, does it deviate in the wind direction? There would be no time lag as the bullet is accelerating the whole way. Would the BC of that object(hi or lo) even matter given any acceleration(like a rocket)?

 

[Bart B]

Mike, you made some good points. Yes, drift and deflection are two different things that are often intermingled.

I think "drift" should be what's caused by the dynamics of the bullet and not that of any external force on it. Spin drift's caused by the gyroscopic stuff bullets have.

"Deflection" is caused some external force that moves the bullet. Gravity pulls it down. Wind blows it in all directions, speeds it up, and slows it down; mostly in the horizontal plane. Colder/thicker air slows it down more than warmer/thinner air.

If a bullet's dropped 100 yards above ground, it'll fall down:

* in different orientations depending on its axis angle upon release and probably end up going base first at the last part of its travel. If the wind's blowing, it'll move horizontally with the wind in varying amounts depending on its orientation to the wind direction; less for a given drop distance as it speeds up so its curve will be the reverse of fired bullets that slow down to impact. Its speed will increase until it reaches maximum that may well vary.

* in a point down orientation of directed that way and spun fast enough to stabilize it. It'll also deflect horizontally depending on its resistance to the wind's force. Its spin axis will stay aligned with its fall axis until impact.

If a bullet's shot horizontally from a barrel properly spin stabilized, it'll:

* fly through its trajectory with its long axis parallel to that trajectory until it starts transcending the sonic barrier; depending on its imbalance, it'll take off in some direction other than a normal trajectory and it'll slow down faster. A cross wind will move it horizontally and its spin axis will follow the combined fall due to gravity and deflection from wind forces. If the wind stops while it's angling off to the side of the line of sight, the bullet will maintain that direction and fall normal rates until some other external force moves it.

* There'll be a small amount of spin drift in the direction of the rifling.

* For a right hand twist, the bullet will tend to deflect lower towards 4 o'clock with a wind from the left, higher towards 10 o'clock with a wind from the right. It's much more noticeable with bullets going near the speed of sound.

======================

Any unbalance the bullet has upon exit from the muzzle will deflect it from the bore axis in the direction of the heavy side. It'll wobble/nutate enough to increase drag and strike lower on target than these leaving at the same muzzle velocity but are perfectly balanced.

Anytime the bullet's long axis is not parallel to its trajectory, it'll have more drag