how to move the gun with the camera

Hi blender artists, i’m on my way to make my first sandbox game :smiley: and my first one of all. and i want to have an aim system like in GTA games, when you aim at a target say the sky and the character moves the gun at that point, already have a camera that moves with the mouse, up and down, side to side, and a second camera that acts like an aim view, but i want my character also move the gun and track it to the camera movement. How can i make the mouse movement afect also the arm of the character?
here’s a preview of the game
http://postimg.org/image/mwhan69lb/

and here the view of the second camera

http://postimg.org/image/pw6g0ouw1/1583d441/