Range Report wind drift - bullet weight and speed consideration

[turbo54]

Not trying to open a huge can of worms here- just a little one.

Im trying to figure out what advantage a lighter, faster bullet has over a heavier, slower one. The way I see it, you can visualize wind drift by figuring out time of flight, then figuring out what height you need to drop a mass from to give that mass the correct amount of time in the air. Then drop the bullet in question (while wind is blowing). This should show you how far the bullet will move from wind, right?

Let's look at a 308 example...

A 180gr bullet going 2500fps would seem to
have a large advantage over a 155 going 2850. The slower bullet is going 12% slower, but is 16% heavier. I've never seen published drag numbers 90* to a bullet...side profile. Because I've never seen those numbers, I can only assume the key element to bucking the wind is simply a bullets own inertia, which is just weight.

What am I missing here, if anything?

Just reconsidered... the muzzle velocity isn't what to look at...its time of flight. According to my logic, the bullet weighing 16% less would need a minimum of 16% less time of flight to equal the windage characteristics of the heavier bullet.

...still wondering about the drag numbers for bullets in side view...

 

[redbullitt]

Re: wind drift - bullet weight and speed consideration

I am a heavy fan myself...

 

[Guest]

Re: wind drift - bullet weight and speed consideration

One thing many never consider is where the bullet is flying.

Many times I've had my .308 an 300wm both out an ready. Last trip to 1K yds I needed 7.5 IPHY of wind on the 308 with 37 IPHY of up. to hit a MOA size target. The 300wm took 0 wind and 22.5 IPHY of up to punch the same target. Picked up the .308 again with the same settings as before and was on target still. The difference was where the bullet path was right then. I find the closer to the ground I can keep the bullet the better I seem to do.

 

[PaulStafford]

Re: wind drift - bullet weight and speed consideration

Bryan Litz's Applied Ballistics book covers this topic really well, from a theoretical standpoint and practical examples, in chapter 5.

The main indicator of wind insensitivity is the lag time, defined as actual_time_of_flight minus time_of_flight_in_vacuum. The shorter the lag time, the less sensitivity to wind.

Obviously, both BC and MV affect lag time. However, it is more difficult to reduce lag time with MV than BC.

Bryan says it best with this sentence " given a constant level of initial energy, the heavier bullet ends up with less lag time because the increased BC decreases lag time more than the velocity depression increases it."

And so, the heavier bullet wins.

It might be interesting to do some work with the JBM calculator to see this effect.

 

[damoncali]

Re: wind drift - bullet weight and speed consideration

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body">
Im trying to figure out what advantage a lighter, faster bullet has over a heavier, slower one. The way I see it, you can visualize wind drift by figuring out time of flight, then figuring out what height you need to drop a mass from to give that mass the correct amount of time in the air. Then drop the bullet in question (while wind is blowing). This should show you how far the bullet will move from wind, right?
</div></div>

That's not how it works. From the bullet's point of view, there are two forces. One is gravity, and isn't terribly important here. The other is drag, which points opposite of the bullet's point. There is no sideways force from the wind, only backwards from the air flowing around the bullet.

This is because when you shoot in the wind, the bullet yaws into the wind like a weather vane, and flies crooked from the shooter's perspective. This is what spin-stabilized projectiles do.

Since the bullet only sees force pointing backwards, and backwards is now now crooked (from the shooter's perspective), it's own drag pushes it to the side a little. The more wind there is the more crooked the bullet will fly, and the more the drag force will point sideways.

That's why wind deflection only depends on BC, and not some sideways drag coefficient. The very same drag that defines a bullet's drop is what determines a bullet's wind deflection.

 

[ida83704]

Re: wind drift - bullet weight and speed consideration

Shoot the highest BC bullet you can push.
BC=wind shedding.

Check this.
A friend an I were shooting 1450 yards.
I was shooting a .300WM, he had a .338LM.

My 190SMK@3000fps needed 16moa of wind.
His 300VLDhybrid@2750 needed 7 moa of wind

I was 49moa up he was 45 moa up from 100yards

 

[turbo54]

Re: wind drift - bullet weight and speed consideration

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: damoncali</div><div class="ubbcode-body">That's not how it works. From the bullet's point of view, there are two forces. One is gravity, and isn't terribly important here. The other is drag, which points opposite of the bullet's point. There is no sideways force from the wind, only backwards from the air flowing around the bullet.

This is because when you shoot in the wind, the bullet yaws into the wind like a weather vane, and flies crooked from the shooter's perspective. This is what spin-stabilized projectiles do.

Since the bullet only sees force pointing backwards, and backwards is now now crooked (from the shooter's perspective), it's own drag pushes it to the side a little. The more wind there is the more crooked the bullet will fly, and the more the drag force will point sideways.

That's why wind deflection only depends on BC, and not some sideways drag coefficient. The very same drag that defines a bullet's drop is what determines a bullet's wind deflection.</div></div>

Fascinating... And completely non-intuitive in regards to my understanding of physics. I am an engineer, but NOT an aero-guy. Can you direct me to something for further reading?

 

[turbo54]

Re: wind drift - bullet weight and speed consideration

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ida83704</div><div class="ubbcode-body">Shoot the highest BC bullet you can push.
BC=wind shedding.

Check this.
A friend an I were shooting 1450 yards.
I was shooting a .300WM, he had a .338LM.

My 190SMK@3000fps needed 16moa of wind.
His 300VLDhybrid@2750 needed 7 moa of wind

I was 49moa up he was 45 moa up from 100yards

</div></div>

What I'm hearing pretty much agrees with what I've thought all along...essentially, gravity is easy to compensate for, so shoot the biggest, fattest, highest BC bullet you can launch at a reasonable velocity. For me, thats the .308 180SMK @ 2550fps.

 

[turbo54]

Re: wind drift - bullet weight and speed consideration

damoncali:

From what you're describing, it seems as if a bullet would "keyhole" through a target in a good stiff wind, even though it has been properly stabilizied by its spin.... Correct?

Do you have any "feel" for how much wind it would take for this keyholing to become obvious for a .30 caliber projectile at about 2550fps MV at a range of about 600 yards?

 

[Greg Langelius *]

Re: wind drift - bullet weight and speed consideration

LL uses a term, "Maximum Ordinate", which means, generally, the peak height of the trajectory.

What many know is that winds aloft will vary with altitude, and that generally, the higher they are, the more velocity they have.

This could be considered to attribute less drift to flatter trajectories, as they traverse regions where crosswinds should be less intense.

I'm not quite ready to bet the farm on that assumption, but I do think it bears consideration.

There are two kinds of stability; static stability (that of a spinning top), and dynamic stability (that of a weathervane). Each operates independently.

The magnitudes of drop and drift are products of forces acting over time, and the two factors are multiplicative.

Greg

 

[Bugholes]

Re: wind drift - bullet weight and speed consideration

A consideration usually not mentioned is the bullets inertia as a ballistic factor. For example, if a 160 grain 7mm bullet had same BC as a 210 grain 30 caliber, ballistic calculators would assume the wind drift would be the same. I imagine the heavier bullet would drift less because it has more energy, more inertia. If you had 2 balls of equal size and shape, one made of styrofoam and one made of steel- and dropped them from elevation in a windstorm, would you expect them to land in the same place?

Does anyone know if weight is calculated in BC? Or is it merely a measurement of efficiency of form?

 

[damoncali]

Re: wind drift - bullet weight and speed consideration

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body">damoncali:

From what you're describing, it seems as if a bullet would "keyhole" through a target in a good stiff wind, even though it has been properly stabilizied by its spin.... Correct?

Do you have any "feel" for how much wind it would take for this keyholing to become obvious for a .30 caliber projectile at about 2550fps MV at a range of about 600 yards? </div></div>

They do, in fact "keyhole" - sort of. The wind speed is so small in comparison to the muzzle velocity (say, 10fps compared to 3000 fps) that the yaw angle (off the line of flight) is very, very small. So the "keyholing" is really just an imperceptible "ovaling" more than a sideways hit. You'll never be able to notice it at the range.

I'm not sure what happens if you shoot in a several hundred mile per hour wind (like out the side of an airplane) - Robert McCoy's book describes firing a 50 caliber machine gun out an airplane traveling at 760 fps and geting a resultant initial yaw of 14 degrees.

Point mass ballistics calculators make an assumption that the yaw is small - if you start to have larger bullet yaw, you really need to use a more complex model (the so called 6DOF model) which is beyond the reach of the average bear, as it requires voluminous data on the projectile which can only be had at great expense (e.g. military budgets).

 

[damoncali]

Re: wind drift - bullet weight and speed consideration

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bugholes</div><div class="ubbcode-body">A consideration usually not mentioned is the bullets inertia as a ballistic factor. For example, if a 160 grain 7mm bullet had same BC as a 210 grain 30 caliber, ballistic calculators would assume the wind drift would be the same. I imagine the heavier bullet would drift less because it has more energy, more inertia. If you had 2 balls of equal size and shape, one made of styrofoam and one made of steel- and dropped them from elevation in a windstorm, would you expect them to land in the same place?

Does anyone know if weight is calculated in BC? Or is it merely a measurement of efficiency of form? </div></div>

You would be correct that mass impacts wind drift. But it is already factored into the BC. That's the convenient thing about BC's: they take into account both geometry and mass.

 

[Fred_C_Dobbs]

Re: wind drift - bullet weight and speed consideration

In this PDF from his Applied Ballistics web site, Litz calculates that a bullet with a BC lower by 0.10 would need an additional 496 fps MV to have the same wind drift as the higher BC bullet.

 

[Greg Langelius *]

Re: wind drift - bullet weight and speed consideration

Weight/mass is a factored into BC. Bullets sharing BC and velocity exhibit identical ballistics.

 

[turbo54]

Re: wind drift - bullet weight and speed consideration

Guys: I applaud you all on this thread. Very informative and also very informational! No name calling or ridiculous bickering that I've come to expect on the hide. Thank you! Feel free to continue tossing in information and thoughts.

 

[vman]

Re: wind drift - bullet weight and speed consideration

Another good thing to take into consideration where shooting the heavier projectiles over the light projectile, is that you should have better barrel life due to the reduced MV of the heavy projectile.

 

[damoncali]

Re: wind drift - bullet weight and speed consideration

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body">
Fascinating... And completely non-intuitive in regards to my understanding of physics. I am an engineer, but NOT an aero-guy. Can you direct me to something for further reading? </div></div>

Sorry - just saw this. Two great books:

Robert McCoy's Modern Exterior Ballistics (only for engineers and math geeks - the math is no joke). This is basically a college level overview of exterior ballistics engineering. Be warned - there are lots of typos, but the errata can be found online if you Google it.

Bryan Litz's Applied Ballistics for Long Range Shooting. This has much less math but lots of good explanations. More practical than McCoy's book, but the one thing it doesn't do is tell you *how* to calculate trajectories. Non-engineers will find this book accessible and useful.

 

[ida83704]

Re: wind drift - bullet weight and speed consideration

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bugholes</div><div class="ubbcode-body">Does anyone know if weight is calculated in BC? Or is it merely a measurement of efficiency of form? </div></div>

all you need to calculate anything is:

BC
Muzzle Velocity
Un Corrected Pressure
Air Temp
and minimally, humidity

If sombody is talking humidity, they are really good or a piker, it is easy to spot the diff.

 

[MitchAlsup]

Re: wind drift - bullet weight and speed consideration

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ida83704</div><div class="ubbcode-body">

all you need to calculate anything is:

BC
Muzzle Velocity
Un Corrected Pressure
Air Temp
and minimally, humidity </div></div>

You forgot the "drag model".

 

[turbo54]

Re: wind drift - bullet weight and speed consideration

I guess I understand now why so many 308 guys are using the Lapua 155gr scenar. The BC is HIGHER than the 180smk, but you can drive them 350fps faster. Ballistically, it seems there are ONLY advantages to the 155 scenar over matchkings.

All this time I had thought weight was an additional benefit over BC.

 

[Fred_C_Dobbs]

Re: wind drift - bullet weight and speed consideration

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body">I guess I understand now why so many 308 guys are using the Lapua 155gr scenar. The BC is HIGHER than the 180smk, but you can drive them 350fps faster....</div></div>
I guess it depends on the source of the BCs. In <span style="font-style: italic">Applied Ballistics</span>, Litz calculates the averaged G1 BC for the 155 Scenar at 0.462 and the 180 SMK at 0.482. In the banded velocity BCs, the Scenar is higher @3000 fps but the SMK beats it from 2500 on down. By 1500 fps, the Scenar is down to 0.414 but the SMK is still at 0.480.