Auto aim and shoot turret issues

Hi,

i found JS of automatic aim and shoot turret and convert it to C#.

this is what i got(with some improvement of mine to smooth the movement):

  using UnityEngine;
     using System.Collections;
     
     public class TurretController : MonoBehaviour {
     
         public Rigidbody bulletPrefab;
            private Transform target;
         private GameObject bullet;
         private float nextFire;
         private Quaternion targetPos;
     
     void OnTriggerEnter(Collider otherCollider) {
         if (otherCollider.CompareTag("airplane"))
             {
                 Debug.Log ("in");
                 target = otherCollider.transform;
                 StartCoroutine ("Fire");
             }
         }
      
     void  OnTriggerExit(Collider otherCollider) {
              if (otherCollider.CompareTag("airplane"))
             {
             Debug.Log ("out");
             target = null;
             StopCoroutine("Fire"); // aborts the currently running Fire() coroutine
             }
         }
      
     IEnumerator Fire()
     {
         while (target != null)
          {
             nextFire = Time.time + 0.5f;
             while (Time.time < nextFire)
             {
             // smooth the moving of the turret
             targetPos = Quaternion.LookRotation (target.position);
             transform.rotation = Quaternion.Slerp(transform.rotation, targetPos, Time.deltaTime * 5);
             yield return new WaitForEndOfFrame();
             }
      // fire!
             Debug.Log ("shoot");
             bullet = Instantiate(bulletPrefab, transform.position, transform.rotation) as GameObject;
             //bullet.rigidbody.velocity = transform.forward * bulletSpeed;
         }
         }
     }

it's work fine when i place single turret on the screen. when the plane enter the collision area it trigger the aiming and get shot. problems starts when i instantiate several turrets in my scene. (they placed in line along Z axis) then they all act weird. basically they all move together get stuck on some point and shot there. the first and second turrets in line kind of work OK. i thought it because they "trigger" each other so i placed them in another layer changed layers collision settings so object in the same layer will not collide with them self. (but since i use trigger i'm not sure it effective ).

so, any advice what could be wrong or how can i implement a better turret?