I am a fantastic sniper in any game. I just love the weapon. I have halo 3 as well. Basically you will want a lower... err can't think of the word right now... a lower turn speed (the numbers that you change from 1-10 in the controller settings). If you are used to it being rather high like me (8-10), you may want to drop it down to 3-4 for sniping. I use 8-10 and still do good sniping, but i'm used to it. Then you have to practice keeping your tiny circle near their head before zooming. If they are 10 meters (or 30ish feet) try to get your tiny aiming reticule in the general head area, then zoom and focus. Also, never stand still. I know it can be hard getting used to running and sniping, but unless you have a great hiding spot, you gotta move.