gun sway, gun swing, gun offset

i am having a problem trying to find how to make a gun sway/gun swing/gun offset/gun rotation script that makes the gun sway in the direction the mouse is moving such as in these videos http://www.youtube.com/watch?v=WYHJ8j6aEtc i have looked everywhere for months but cant find anything does anyone know how to do this please need help ??????????