I'll put this simple. You need ACOG (for power), Stopping Power (more power), an M40A3 (for accuracy), and Steady Aim (aids accuracy). Get them in the middle of your crosshairs and press triangle IMMEDIATELY followed by R1 (convert controls to 360 if needed). Do this properly and in one swift motion and it should seem as though your gun disappeared for a moment and reappeared and shot (you should not see your secondary weapon come out). This causes the bullet to come out more precise overall.