Rotate child of FPS when camera moves on Y-axis

I have a First person controller with a gun as a child. When the FPS looks left or right, the gun moves with it so the gun is pointing in the direction the FPS is looking. Now, when the FPS looks up or down , I want the gun to rotate forward or back (to simulate the FPS looking down the sights of the gun). What FPS or Camera event can I tap into to achieve this?