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Wind drift and velocity

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  • Vaquero

    Moving stuff to the gas prices thread.....
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    This is too good to not have it's own thread. Thanks Ben.

    It makes my heart dance with joy to hear somebody speak up who actually understands this. For everybody else -

    Wind deflection is a function of the rate of velocity loss. It has nothing to do with velocity or flight time.

    Anybody can do an experiment to prove this with any bullet. Plug any bullet into any decent ballistics calculator. Launch the bullet at 975 fps and at 1300 fps and check wind drift at 200 yards with a steady 10 mph crosswind.

    Example? Plug in those numbers for a random bullet (say, a .243 115 Berger VLD) into the JBM drift calculator that everyone uses and trusts. While the flight time of the faster bullet is ~20% less than the slower bullet, the faster bullet drifts 8.3 inches while the slower bullet drifts only 4.1 inches.

    It's possible to spend a lifetime shooting successfully in competition but never grasp the concept that......speed and flight time have absolutely ZERO to do with wind drift.

    Drift is a function of the rate of velocity loss. Above about 2100 fps, speed, rate of velocity loss, and drift track with each other. It's easy to get the impression that more speed equals less drift all the time. That's completely wrong. I am astounded at the number of experienced long-range rifle shooters who do not understand this. They're the ones who, deep down, don't really know why a 168SMK out of a .308 is such a joy out to 600 yards but goes completely to hell somewhere between 800 and 1000. They talk about "the transonic region" and do some hand-waving but, all too often, they don't really understand what's happening.

    If you want to see perfectly linear wind drift all the way to the target, bullet velocity must be kept over 2000 fps for the entire trip.

    Want an interesting illustration of that?
    • The .17HMR with a 20 grain bullet, according to the factory, launches at 2375. Plug that into the JBM calculator and the drift at 200 yards in a 10 mph crosswind is 17.8 inches.
    • OTOH, load a round of Eley Tenex .22LR into a barrel short enough to get the muzzle velocity down to 975 fps and the wind drift at 200 yards in that same wind is 14.9 inches.
    The reason those numbers seem so weird is that the poor 17 HMR falls below 2000 fps before it goes 100 yards and is only going about 1300 fps at 200. That 2000 fps to 1300 fps range is where all bullets lose their minds and start to drift in the wind to a ridiculous degree, compared to what they were doing when they were traveling over 2100.

    Of course, shooting subsonic rimfires at 200 yards invites elevation errors. The same is true if you want to stretch your 300BO with subsonic loads out to several hundred yards. So it's a balancing act and there are plenty of ways to weigh up what you consider important and what you don't.

    But the oversimplification of wind drift to "more speed is better" is a personal pet peeve. I really wish people would learn this stuff. I have multiple pieces of correspondence from a ballistician at Sierra from when I was a kid and I noticed things I didn't understand about all this in their published ballistics charts. (Us old guys remember when the reloading manuals were so thick because they had a hundred plus pages of numbers in dense charts in the back, all to describe bullets in flight.) They caught onto the "Wind drift has nothing to do with speed; it's a function of the rate of velocity loss" back when they were compiling the tables for the first edition of the Sierra manual. He told me fascinating stories about how they were surprised to see the .45-70 outperforming other, faster cartridges when it came to wind drift at intermediate ranges.

    This was new thinking 60 or 70 years ago but that was a long time ago. There's no reason for anyone to still be confused about it.
    Guns International
     

    orbitup

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    benenglish

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    That's an excellent illustration. Now lets plug in numbers that do a better job of showing what happens when that bullet enters transonic velocity ranges. This is from the JBM simplified trajectory calculator that I think everybody can trust.

    The closest bullet in the JBM database to the one you used would be the 20 grain Hornady .17 V-Max. Here's what the trajectory looks like when you launch it at a velocity that's just above the transonic range, 2100 fps.

    1648822823748.png


    That's 12.9 inches of wind drift at 200 yards in 0.359 seconds of flight time.

    Now, lets launch the same bullet from just below the transonic range, at 975 fps.

    1648823051603.png


    That's 10.7 inches of wind drift in a flight time of 0.676 seconds.

    Roughly speaking, the faster bullet drifts ~20% more while it spends (+/- 4%) half the time in the air.

    The basics remain the same for every bullet, no matter how fast it's launched. Once a bullet drops below ~2100 fps, wind drift numbers are no longer tracking velocity. They never were but this is the range where it becomes obvious.

    Shooters who shoot targets sufficiently close that their bullets are always traveling above ~2100 fps enjoy very intuitively predictable wind drift. They can dial in the same number of clicks on their rear sight for the same wind with every identical change in distance. As long as the wind is steady, compensating for it is easy.

    But as velocity drops under that level, that nice, (seemingly) linear drift goes completely to hell.

    That remains the case until the effects of the transonic region are left entirely behind, which doesn't happen until the bullet is traveling less than 1000 fps.

    Why do the calculators produce all this unintuitive output? Why are the skilled spotters at Palma matches worth their weight in gold (especially back when everyone was shooting a .308)?

    Because wind drift was never related to bullet velocity at all.
     

    oldag

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    No, the example I posted showed the faster bullet drifts less in that specific case.
     

    benenglish

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    No, the example I posted showed the faster bullet drifts less in that specific case.
    Yep. It sure does. In that specific case and others like it, it happens a lot. If fact, it happens so much that many shooters get the erroneous idea that, somehow, bullet velocity and wind drift are related.

    The guys that shoot Palma, BPCR Silhouette, pistol silhouette, and a few other disciplines know better and have for decades. I'm just happy to expose more folks to that reality.
     

    popper

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    Funny, I went through the code for one of the well known calculators, used gravity in the wind drift calc. It's actually the rotational momentum that fights wind drift and gravity effects, not fps. Center of force and lateral cross section. Yes, passing through transonic region is a mess.
     
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