The rendering distance of 1km is not high enough.

The longshot achievement has a minimum range of 950 meters, you only get a window of 50 meters to hit someone.

However, you can see muzzleflashes and tracers at 1.3km, but not the actual player model, and though your bullets do render and hit environmental object like buildings at this range, the player does not.

I think they should up the render distance, because if I can manage to have someone stand still long enough at 1.3km to hit them with a mosin round (about a second and a half of hang time) I think I should be able to hit that without being limited by the rendering. (It's also jarring to see helicopters and such pop in and out at this range too)