How do I use the scope on the M21 sniper rifle?

1. In weapon view, peer through your sight and line up your target so that he is in the center of the sight. 
   
2. Zoom in/out using the +/- numpad keys until the horizontal hairs are aligned with the top and bottom of your target. This sets your sight to the range of the target. Calibrating your sight in this way ensures that your bullet will be delivered to the point indicated by the crosshair at the center. Provided your target does not move, you can now target a different area with a reasonable degree of certainty. Use this method if you need to cause the most minimum of attention and take down an enemy with a one-shot-one-kill headshot.