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Old December 1, 2012, 06:29 PM   #52
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Join Date: December 20, 2008
Location: Somewhere on the Southern shore of Lake Travis, TX
Posts: 2,479
I have been doing some thinking about how to make wind drift more understandable.

When you are driving a car down the highway and you spit out of the window, it looks like the wind outside the car takes whatever you spat out and blows it backwards.
But there is no wind out there, the "wind" you feel when you stick your hand out the window is due to the motion of the car. What you really saw was the velocity of the spit, which started off going forward just as fast as the car was going, slowing down and your car, which was not slowing down, leaving it behind. That spit actually went in a straight line from a stationary reference point.

OK, instead of the air moving, let's assume the shooter and target is what's moving. We have a shooting station on the deck of a ship and a target on another ship 200 yards away and both ships are traveling parallel to each other in the same direction at 10 mph in a dead calm. The ship's speed makes this dead calm look like a 10 mph crosswind.
The bullet obviously travels a straight line because the wind is actually calm.
Because the target is moving, the shooter has to lead the target just like when shooting at flying birds with a shotgun.
But the shooter is also moving and that motion gives the bullet the lead needed to hit the target. Gunners will tell you that when you are strafing ground targets out of the side of a helicopter gunship, you have to lead those ground targets just as if the helicopter was stationary and the ground was moving under you.
So these two things cancel each other out, as long as the bullet does not arrive late. The later the bullet arrives, the more behind the target the bullet hits.

Now consider a .17 HMR with a muzzle velocity of 2550 fps.

At 2550 fps, it takes .235 seconds to travel 200 yards, but the bullet slows down, at 100 yards it's only going 1900 fps and at 200 yards, it's down to 1380 fps and as a result, it takes it .324 seconds to go 200 yards. Subtract .235 from .324 and we see that the bullet arives at the target .089 seconds late. Since 10 mph is 14.667 fps, 14.667fps X .089 seconds is 1.3 ft or 15.6 inches more that the target moved while waiting for the bullet to get there.

Now let's look at a Whisper 300 shooting a 220 grain hpbt with a BC of .608 with a muzzle velocity of 1040 fps.
At 1040 fps, it should take .577 seconds to reach that target.
But at 100 yards, this bullet has slowed to 1002 fps and at 200 yards it has slowed to 970 fps and as a result it took it .598 seconds to go 200 yards.
This bullet is .021 seconds late and .021 seconds is enough time for that moving target to go an extra 3.7 inches.

The more a bullet slows down during its flight, the later it gets to that target and the more it misses the bullseye of that moving target.

Now here comes something that's totally counterintuitive. Let's say the bullet speeds up during its flight. Now the bullet gets to the target early instead of late and instead of underleading this moving target, we are overleading the target and the bullet seems to drift against the wind instead of with it. Of course, bullets never speed up during flight unless you are shooting rocket assisted artillary rounds.
I dunno, maybe rocket propelled projectiles actually do climb the wind instead of drifting with it while the rockets are accelerating the projectile.
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