but it's insanely difficult.

At that difference, you need have your range spot on, you need to calculate any winds or cross winds, you need to make sure all of your bullets are exactly the same, including charge (so essentially you are reloading yourself, as factory loads tend to have some variation), so that holding the reticle steady at that range and pulling the trigger with little movement is the easy part.

I can't imagine how tough it would be to be accurate at the range in a combat situation. That's probably why snipers usually have incredible spotters.