[adrianpike]

Both - the firearm has to be incredibly precise, but you also have to take into account wind and air density to get that much accuracy, and a big portion of marksmanship is accounting for those. Knowing that you'll have a temperature change over a body of water, and how to adjust for that, for example.

[spywaregorilla]

But I can't imagine you're dynamically adjusting each shot for these changing factors every time. You'll almost certainly be holding the aim fixed once you've figured it out to be approximately correct, and firing against natural variance in the variable conditions.