Instructions
Start "Call of Duty 4: Modern Warfare" on your game console and begin playing the desired mission. Pick up the first MP5 you find if you don't have one already.
Press the "Start" button on your controller to pause the game and select "Options" from the "Pause" menu.
Scroll down to "Aim Assist" and select "Enabled." Press "Start" again to resume the game where you left off. The following step must be accomplished in quick, sequential order.
Pull the left trigger before shooting enemies to automatically center your sights on your enemy's chest (this is the "Aim Assist" feature). Quickly move the right stick up just a hair after the sights have been centered and pull the right trigger to shoot your enemy in the head with your MP5.