I'm going preface this by saying I've reported this bug once on the old forums when Dirty Bomb was published by Nexon, since then the thread is gone and the issue is still very much present.
What is the bug? Well, basically aiming is flawed, it seems like you can only aim so precise, the game prevents you from doing true pixel perfect sniping, it's quite noticeable when playing at a (very) low sentivity, such as 2.4 ingame@1600dpi and scoped in, the issue is also present when firing from the hip, but as you aren't using scoped sensitvity or FOV, it's almost impossible to notice. And just for clarification, this ISN'T a mouse smooth or acceleration issue. It's more like the integer or float used to represent where you're aiming might only go so precise, so it's like your aim is snapping to some sort of grid.
My words don't do the issue justice, so please just watch two short videos I made to show the issue.
https://www.youtube.com/watch?v=ad8tWUPBhsc (the issue)
https://www.youtube.com/watch?v=IRjqzhS6Ohc (how aiming should look)
Anyway, thanks guy, I'll be keeping an eye on the thread, as I truly think Dirty bomb is an outstanding game, and would like to see this bug fixed.