Gun movement in 3rd person character using mouse.

Hey there! I'm making a game like GTA, wherein I'm letting my character hold gun in 3rd person using Inverse Kinematics. The problem is how to move the hand of the character when in gun mode (that is , moving the gun along with it to aim at a particular object) using the mouse. I've tried many solutions from the net but none of them seem to work. Any help? A script in C# would be helpful. Thanks in advance.