If you're missing by that wide of a margin it's probably because you're moving while aiming. That will take a huge toll on your character's accuracy.
But you should also notice that sniper jobs are naturally difficult in Rage because there are no aiming aids of any kind (like Halo, for example) and the bullets use per-polygon collision detection. That means you actually have to
hit somebody to hurt them - you can't just aim in their general direction, pull the trigger, and expect a kill.