Moving around a weapon on screen using OnMouseDrag

Hey dudes. I'm creating a first person game where the player can move around his weapon on the screen. Basically you're just supposed to hold down a button (so far I'm using LMouse but RMouse will be the final button) and move the mouse and the camera will freeze and the weapon will move instead of the camera.

I've gotten the weapon to move and all using a plane with an OnMouseDrag-function but first of all it's moving way too slow (I want it to move at the same speed as the mouse) and secondly it's jumping back and forth 1 frame at a time when I move the character around while holding the mouse button down.

Any tips on how to straighten things up? Is my method using the plane void?

Here's the code: #pragma strict

 var cam : Camera;
 var controller : GameObject;
 var sword : GameObject;
 var swordDrag : boolean;
 
 internal var gameSens : float;
 internal var screenPosition : Vector3;
 
 function Start()
 {
     gameSens = cam.GetComponent(MouseLook).sensitivityY;
     Screen.lockCursor = true;
     screenPosition = cam.WorldToScreenPoint(transform.position);
 }
 
 function OnMouseDown()
 {
     swordDrag = true;
 
     cam.GetComponent(MouseLook).sensitivityY = 0;
     controller.GetComponent(MouseLook).sensitivityX = 0;
 
     
 }
 
 function OnMouseUp()
 {
     swordDrag = false;
     cam.GetComponent(MouseLook).sensitivityY = gameSens;
     controller.GetComponent(MouseLook).sensitivityX = gameSens;
 }
 
 function OnMouseDrag()
 {
     if (swordDrag == true)
     {
     //Read the mouse input axes
     screenPosition.x += Input.GetAxis("Mouse X");
     screenPosition.y += Input.GetAxis("Mouse Y");
 
     //move the object to the world position to not change screen position
     sword.transform.position = cam.ScreenToWorldPoint(screenPosition);
     }
     //else return;
 }