Shotgun range?

Hello, I'm trying to make my cube to shoot a shotgun. The problem is that I want the shotgun to have a maximum range where everything(zombies, soldiers) that is before the maximum range to explode. My problem is that my cube is moving on the x and y axis. So I wouldn't know how to set the limit on my x and y axis. Also destroy the object once it goes further than its minumum range. Its a 2d game being made in c#

private Transform myTransform; public float shootingSpeed = 5f; public float specialgunSpeed = 10f;

 // Use this for initialization
 void Start () {
 myTransform = transform;
 }
 
 // Update is called once per frame
 void Update () {
     float amntSpeed = shootingSpeed * Time.deltaTime;
     myTransform.Translate(Vector3.up * amntSpeed);
     myTransform.Translate(Vector3.forward * amntSpeed);
     
     if(myTransform.position.y >= 8f){
     Destroy(this.gameObject);
     }
 
 }
 //Special guns are shotguns, rpgs, etc
 void SpecialGuns(){
     float amntSpeed = specialgunSpeed * Time.deltaTime;
     //This is the guy I worry about.
     myTransform.Translate(Vector3.Max (3f,3f) *amntSpeed);    
 

}