Here is the code:

Code:

---

define barrel_handle, skill1;

action ent_turret
{
	my.barrel_handle = handle(ent_create("turret_barrel.mdl", my.x, NULL));
	
	while(my) {
		
		you = ptr_for_handle(my.barrel_handle);
		
		var  vec_to_ent[3];
		var  ang_to_ent[3];
		
		// get the direction vector turret->camera and its angles
		vec_diff(vec_to_ent, camera.x, my.x); 
		vec_to_angle(ang_to_ent, vec_to_ent); 
		
		// first rotate the turret's body
		ang_rotate(my.pan, vector(ang(ang_to_ent.pan - my.pan),0,0));
	
	        // now rotate the barrel
		vec_set(you.pan, my.pan);
		ang_rotate(you.pan, vector(0,ang(ang_to_ent.tilt - you.tilt),0));
		
		
		
		wait(1);	
	}	
}

---

As you can see, the calculations of the code above will not work with arbitrary orientations.
However, I have no idea what has to be changed to get it working.