Bucking wind vrs BC

[bullet]

Hmmm! So, the 220 is slower than the 200 therefore has longer TOF but deflect less.

Rovert, I'm confused

+ ONE, Desert Fox is not the only one confused on this one.

 

[Greg Nolan]

It looks like the 220 gr. bullet has a 16% advantagein BC over the 200 gr. bullet and my calculations show 16% less drift . I may be off base but I'm trying to learn and gain understanding.

 

[ROVERT]

The lag time is what makes the difference. To determine the effect that wind can have on a bullet you must look at the additional amount time that the bullet is in flight because of drag. This means subtracting the amount of time from ToF that the bullet would require to travel the give distance in a vacuum. When it is in a vacuum it is not affected by wind or drag. Therefore that component of the total flight time is not important to wind drift.

Give me any B.C. and muzzle velocity and I can calculate the wind drift at a given distance. I don't need to know the mass of the bullet.

Here's an example:

.308 cal 200 gr. Nosler AccuBond fired at 2800 fps drifts 15.9" at 500 yds with a 10 mph crosswind.
.338 cal 250 gr. Sierra Match King fired at 2800 fps drifts 15.9" at 500 yds with a 10 mph crosswind.

The SMK has more mass and is fired at the same velocity but drifts just as much. Why? Because the NAB has a B.C. of .588 and the SMK has a nearly identical B.C. of .587. When the ballistic coefficients are the same and the the muzzle velocities are the same, the lag time is going to be the same regardless of bullet mass.

I'm not a great teacher, so I may not be explaining this very well, but it is true.

 

[bullet]

Interesting indeed.

 

[ROVERT]

It looks like the 220 gr. bullet has a 16% advantagein BC over the 200 gr. bullet and my calculations show 16% less drift . I may be off base but I'm trying to learn and gain understanding.

I didn't run those numbers, but I believe it is a coincidence. If you think about it, you could hypothetically drive that 220 gr. bullet at 4000 fps. The B.C. would stay the same (or nearly) but the advantage in wind drift would change drastically.

 

[tddeangelo]

Guys, a fluid is a fluid. Air is a fluid substance. Think of crossing a river. The longer you take, the farther you're pushed downstream, right? More surface area exposed to the current,the more it can bear on you and more it affects you.

Same deal here, far as I can tell.

 

[bullet]

Guys, a fluid is a fluid. Air is a fluid substance. Think of crossing a river. The longer you take, the farther you're pushed downstream, right? More surface area exposed to the current,the more it can bear on you and more it affects you.

Same deal here, far as I can tell.

Very good analogy, really helps you wrap your mind around it without tons of math, thanks.

 

[ROVERT]

There's some good information in this article by Bryan Litz. The paragraph titled Understanding Wind Deflection is very pertinent to this discussion.

http://www.appliedballisticsllc.com/index_files/Understanding_part2.pdf

I've also heard that his book (Applied Ballistics, I think?) is very good.

Most of my reading on the subject has been through New Exact Small Arms Ballistics: The Source Book for Rifleman by Arthur Pejsa. It is good, but pretty heavy on the math. I'm also not sure if he is that much better a teacher than I am.

 

[ROVERT]

Tom, that is a good analogy. It doesn't quite capture the concept of a bullet drifting less even with a slightly longer flight time though. I think this is what is throwing a lot of people off and making them think that there must be other forces at work. Once you understand exactly what component of flight time is important, it all makes sense.

 

[Desert Fox]

I'm still cannot grasp what Rovert was saying here.

If you think that lag time and BC are the only one having an effect on bullet deflection from wind, then you're mistaken. Momentum (Heavy bullet has more mass, therefor has more momentum)bullet shape, velocity and parasitic drag has a lot to do with it also. I'll explain later on. Hard to type in an I phone.

 

[ROVERT]

If you think that lag time and BC are the only one having an effect on bullet deflection from wind, then you're mistaken. Momentum (Heavy bullet has more mass, therefor has more momentum)bullet shape, velocity and parasitic drag has a lot to do with it also. I'll explain later on. Hard to type in an I phone.

There is no momentum without velocity (momentum = mass times velocity) and the the bullet has no velocity in the direction of the wind. Inertia could be argued to play a role here, but the heavier bullet also has more surface area exposed to wind, which negates it's benefit of mass. I believe I mentioned that earlier.

Read what Litz wrote. I feel he stated all of this much more eloquently than I did and he is definitely more qualified to speak on the subject.

 

[bullet]

This is getting good, can't wait for Desert Fox's reply, yep, I am enjoying this.

 

[JD338]

This is good for sure.
It is one of the reasons why I am a fan of heavy bullets.

JD338

 

[bullet]

This is good for sure.
It is one of the reasons why I am a fan of heavy bullets.

JD338

I am a fan of heavy bullets that can be driven fast with good BC because they don't drift much with a cross wind and they retain energy and hit like a freight train. :mrgreen: :mrgreen: OH!! the math behind proving what I said??????? :shock: :shock: :mrgreen:

 

[onesonek]

As far as I know momentum in the pure sense, is in part of ballistic physics as being dicussed here, but not the factor most think or how it is being presented by some. The formulas, or ballistic programs make corrections for such with barametric pressure inputs by increasing or decreasing the BC . We tend to simplify the momentum formula, but remember, weight and mass are not the same thing,,,, altitude or barometric pressure effect the changes. Tempurature and humidity also has it's effects. No program can really counter the effects in real time totally, as it changes even within flight time. Also, MV can alter BC as well. With that,, BC's, MV and current conditions, are all the concerns needed.
However momentum simply figured, is a factor in post impact, but there is a relation to it's cross section in that as well. This is where SD's come into play, intitial unfired and post impact. Also momentum is in the equation of recoil. But that's not what is in question.

Case in point,,,, take a Mono of lighter weight/mass but of equal BC, fired at the same velocity will have the same basic flight characteristics, as a heavier lead core of the same BC, but the momentum is different favoring the heavier.
Post impact weight/cross section, is where I concern myself with momentum.

Moral of the story,,,, trust the ballistic programs, as the mathematicians have got rid of the theory's and hypotheticals,,,, then shoot to confirm to real time as even programers make mistakes. Then too, nothing is perfect due to real time changes. Beyond that all is minutiae,,,,which is why I didn't go into such of my earlier post.

 

[bullet]

. Beyond that all is minutiae,,,,which is why I didn't go into such of my earlier post.

Now onesonek, the above statement is a good signature line. Great summary. :mrgreen:

 

[Ridge_Runner]

any 2 given bullets of the same BC, launched at the same velocity, drift and drop the same, reguardless of weight or diameter, but the heavier bullet will still have the advantage in retained energy.

humidity does effect trajectory but not much, from 0% to 100% humidity the difference amouts to around 2" change of POI at 1000 yards.
RR

 

[onesonek]

any 2 given bullets of the same BC, launched at the same velocity, drift and drop the same, reguardless of weight or diameter, but the heavier bullet will still have the advantage in retained energy.

True,,, I did forget to mention drop along with deflection, I beg forgivenss.



humidity does effect trajectory but not much, from 0% to 100% humidity the difference amouts to around 2" change of POI at 1000 yards.
RR

Yep,,,it ain't much is it. Not enough worth worring about at most normal hunting distance's at least.

 

[Oldtrader3]

...it would seem there's more happening here than just the duration of the flight, air resistance, or in our case lack thereof, or BC & momentum, a lighter bullet is easier to move, are both factors.

The lighter bullet is not easier to move. At least not from the winds point of view. The reason being that the heavier bullet also as proportionately more surface area exposed to the wind.

It really does come down to just flight time, which is purely a function of muzzle velocity and ballistic coefficient. The mass of the bullet is not important.

Thank you Rovert for your surface area explanation. Wind also is a 360 degree vector, two axis (horizontal and vertical component), calculation which Tjen (being an engineer) should know (sines and cosines).

 

[Desert Fox]

Although TOF and BC are the main factors in cutting wind deflection - Momentum, bullet density, bullet velocity and drag is also a contributing factor.

Linear Momentum which is the product of weight times velocity tends to keep the bullet straight. The heavier the bullet, the harder it is for the wind to push it away from it's intended path.

Density- a heavier bullet tends to be denser than the lighter bullet therefore will be influenced less by wind

Velocity and Drag- In order to equal the momentum of the heavier bullet, a lighter bullet has to be push to a higher velocity but, we all know that the lighter bullet will not be able to hold it's velocity at longer distance due to increase drag therefore can be deflected more by the wind.

Bryan Litz in his bullet comparison, as shown in the link provided by Rovert, explained it correctly. To match wind deflection, a lighter bullet has to be pushed to higher velocity when compared to a heavier and bigger caliber bullet. Not sure why Bryan did not include the distance and the tof in his example but I'm pretty sure the 90 grain 223 got to the target sooner than the 220 grain 308. Extend that range further and the 223 bullet will surely be deflected more by the wind than the 308 bullet.

You have to consider the whole flight regime of the bullet when determining how will it behave when subjected to wind. There's no commercial ballistic program out there in the market that can predict with certainty, the effect of wind on bullet. It's all approximation, so you can't rely on them. Empirical measurement and keen observation is much more reliable.

My last 1000 yards score with my 338 Lapua Ackley, shooting the 300 grain SMK, is a good example of why a heavy bullet, at moderate velocity, will fair better on a windy day against faster but lighter bullet.
I find it easy to placed 13 of the 25 bullet into the X-ring. As you can see on the score of shooters, the 338 and the 50's dominated the smaller caliber.