I'm going with Ridge Runner on this. Probably a mirage problem distorting your view. But that shouldn't be a problem anyway, because you SHOULDN"T BE SHOOTING ACROSS A PUBLIC ROADWAY. Don't do it! It's illegal for a reason. Don't be the moron that gets his name on the news for being the one who shot a toddler in its car seat because you were so fixed on the hog in you crosshairs to notice the minivan coming down the road at 60mph.

Secondly, I hate to run up the BS flag, but if you can make 600+ yard shots "easy", then 300 yards is a chip shot. If it's not laser verified, I can't believe it. Your theory about heat rising from the road lifting the bullet is bogus. Take a bullet out to the road, stand right in the middle which should be the hottest part, and drop it. Does it fall or does it rise? Since the bullet creates no lift on its own, the effect of rising air is nil. Besides, even if we'd say that it has some effect on reducing the rate of accelleration toward the ground, at say 2600fps across a 24' wide two lane highway, the bullet spends just 0.0092 seconds over the road. Not enough time to significantly effect trajectory.

Mike