This guide will tell you how to TURN ON GYRO Aiming at Controller in Modern Warfare 2.
With Gyro aiming, you can aim simply by moving your controller in a direction, you can also use the right thumb stick at the same time. You can also fine-tune your aims at specific points. To turn this on, you need to press the Options button, then go to settings and navigate to the controller.
After this, press the R1 button to go to the Advanced tab and navigate down to the gyro aiming, you have to turn it ON. You need to set the vertical and horizontal sensitivity to 100. It is really good for long ranges.