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Author Topic: Velocity and Wind Drfit  (Read 1310 times)

rsterne

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Velocity and Wind Drfit
« on: October 15, 2016, 01:59:27 AM »
I decided to do this thread because of some misconceptions I keep running into regarding wind drift with airguns.... The comments inevitably blame the wind drift that plagues us on the low velocities, and consequent long flight times to the target.... In fact, airguns often operate in a "sweet spot" in terms of low wind drift.... Think I'm crazy?.... Consider this....

The amount of drift is not proportional to the flight time, but rather to the DIFFERENCE in flight time between the real world and what would happen to the same pellet/bullet starting from the same velocity in a vacuum.... The higher the drag, for a given Sectional Density, the quicker the projectile slows down, so the greater the difference between its flight time in air and in a vacuum.... The problem is, that the drag increases many fold as the projectile breaks the "Sound Barrier".... There are several Drag Models, which represent various shapes, and here are a few "drag curves" showing that rapid increase in drag in the Transonic Region (Mach 0.8-1.2)....



If we use a typical drag curve, represented by the G1 Model (the orange line above).... and then use various Ballistics Coefficients, we can plot the wind drift for various muzzle velocities over any range.... I chose 200 yards because that is what is used at the Extreme Benchrest event, just run in Arizona.... Here is what happens for BC's of 0.05, 0.10, 0.20, and 0.40.... which spans pretty much anything we might see in airguns.... These charts are for a 10 mph crosswind, calculated using the JBM Ballistics Calculator.... http://www.jbmballistics.com/cgi-bin/jbmtraj_drift-5.1.cgi



The first thing you will notice is how important the BC is for reducing wind drift.... The biggest influence on BC is the Sectional Density of the bullet, with the shape of secondary importance.... Pellets or roundball might behave roughly like the yellow curve, a chunky slug like a 100 gr. in .357 cal might be like the blue curve, a long thin bullet like a 90 gr. .257 would be something like the orange curve, and a very well designed boattail spitzer might be like the grey line.... The other thing that is apparent is that the drift does NOT get less as the velocity increases above what we usually run with airguns.... In fact, it gets WORSE as we push Supersonic, and you have to reach velocities unheard of with airguns to get back down to the same amount of wind drift we get.... Counterintuitive maybe, but FACT.... Let's concentrate on bullets we might use at the BigBore shoot at the EBR....



The important thing here is to look at the velocity where the wind drift is at a minimum.... For any BC we might use at that event, shot at 200 yards, the least wind drift occurs when using a muzzle velocity of about 900 fps.... True, the curves are pretty flat either side of that (particularly with a high BC).... but you have to push the bullets more than twice that velocity before the drift once again drops to what we achieve with the velocities we already use.... So, when you are cursing the wind when shooting an airgun.... don't blame it on the low velocities we use.... You are looking in the wrong place if you do.... Instead, you need to be looking at a bullet with a better BC.... When shooting around that 900 fps velocity, if you double the BC, you will cut the wind drift roughly in half....

The actual MV you choose will be governed mostly by where your bullet shoots the most accurately.... Anything between 800-1100 fps makes sense, but the closer you stay to 900, the less drift you will have to deal with.... Yes, the trajectory won't be as flat as if you push the bullet at 1050 fps, but gravity is a constant and can be allowed for by zeroing your scope.... The wind is anything BUT constant, so IMO you need all the help you can get....

Bob
« Last Edit: October 15, 2016, 04:00:06 AM by rsterne »

mtnGhost

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Re: Velocity and Wind Drfit
« Reply #1 on: June 13, 2019, 02:42:46 AM »
I wanted to thank you for posting this. It is a fantastic dive into a multidimensional topic that took me some time to figure out on my own.

I started shooting swaged and cast ammo with PCPs in the not too distant past. The areas that I shoot in the mountains are plagued with wind for at least 300 days of the year. I didn't have any understanding at the time of transonic drag or any of the coefficients / principles, so I figured the faster that I could shoot - the less time the projectile would have to hit my target, and I did not consider any of the consequences in doing so.

With that said, I shoot small bore and I don't really shoot anything greater than .30 right now. The thing that is really beginning to interest me as I shoot different types of ammo with different calibers is that BC values do not necessarily translate to some level of guaranteed accuracy. What I'm finding is that for example, shooting at 100y - a .22 slug with a BC of 0.12 can be far less accurate than a .30 slug with a BC of .07, with high degree of consistency. The same .22 slug can be hole in hole in calm winds, yet as soon as there is a wind component of any relevance - the accuracy takes a big dive.

The BC values are somewhat two dimensional. As you say, each projectile seems to have some sweet spot where they're most accurate - and my experience has been that those speeds can be extremely delicate and fragile. Take for example, once again the .30's with the 0.07 BC, shooting lights out at 940fps in calm winds - produce numerous fliers in winds of 15mph or more.. yet, extremely accurate between 890-910fps in 25-30mph winds!

I think the effects of drag in the wind are tremendously exaggerated upon a projectile such as this .30, when it is being shot right on the edge of its max (calm wind) velocity. I really haven't been able to pin down what the "ideal" shape and weight would work best for accuracy in the wind with a small bore PCP (in the goal of achieving long range accuracy). I'm beginning to think that mass is extraordinarily relative for conquering wind, but again based off of my testing thus far, even this theory doesn't seem show that scale is linear.

Ultimately, I'm beginning to think the mass / sectional density has to be over some min-bar in order to shoot well in the wind. .30 cal has been very promising, I have only shot as heavy as 50gr though. I have not had any luck shooting .25 in the wind over 870fps, and that was with my RAW HMX (LW poly). That caliber to me just seems inefficient for long range accuracy in my world.

 



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