Spawn a bullet of a cannon at right position

Hi guys,

I'm still a noob on programming in Unity and I'm trying to make a game with a cannon that can shoot bullets, but right now the bullets are spawned at wrong position: if the cannon is aiming up the bullets are spawned at the bottom of the cannon and if I'm aiming down the bullets are spawned above the cannon. I believe is something with the angle, I guess is inverted, but I still can't figure what is wrong, bellow is my code and thanks in advance for any help!!

 //Shoot.js    
 
 var bulletPrefab:GameObject;    
 var temp:GameObject;    
 var accumulatedForce:int = 0;
 
 var mouseScrollSpeed:float = 5000.0;    
 var rotationZ:float = 0;    
 
 var velX:float = -5.0;    
 var velY:float = -5.0;    
 var newX:float = 0;    
 var newY:float = 0;    
 
 function Update ()    
 {    
     rotationZ = -Input.GetAxis( "Mouse ScrollWheel" ) *     
          Time.deltaTime * mouseScrollSpeed;    
     transform.Rotate( Vector3( 0, 0, rotationZ ) );        
 
     newX = transform.position.x + ( Mathf.Cos(transform.rotation.z ) * velX );    
     newY = transform.position.y + ( Mathf.Sin(transform.rotation.z ) * velY );        
 
     if ( Input.GetKey( "mouse 0" ) )    
         accumulatedForce += 10;    
 
     if ( Input.GetKeyUp( "mouse 0" ) )    
     {    
         temp = Instantiate( bulletPrefab, Vector3(    
                 newX, newY, transform.position.z ), transform.rotation );    
         temp.rigidbody.AddForce( -accumulatedForce, accumulatedForce, 0 );    
         accumulatedForce = 0;    
     }    
 }