I usually play crucible and use snipers in my special slot.
I use kontrol freeks, a long one on my aiming stick.
Also I play on a 24" monitor
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It depends on what kind of player you are. People from the CoD camp tend to have higher sensitivity, but people who came from the halo days tend to have slightly lower sensitivity. I tend to play on 5, it just feels right, not too sluggish, not too loose.