Select prefab from Dropdown UI to instatiate (select ammo type)

I’m a noob, failing at selecting a prefab from a dropdown ui, before instantiating.
Any ideas would be great because I keep hitting dead ends, and trial and error has not been kind to me. Thanks

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class fireProjectile : MonoBehaviour
{
    public Dropdown ammoDropdown;
    public Rigidbody ammo00;
    public Rigidbody ammo01;

    void Update()
    {
        if (ammoDropdown.value == 0)
        {
            //Make ammo00 the projectile
        }
        else
        {
            projectile = null;
        }

        if (Input.GetButtonDown("Fire1") && projectile != null)
        {
                Rigidbody clone;
                clone = Instantiate(projectile, transform.position, transform.rotation);
                clone.velocity = transform.TransformDirection(Vector3.forward * 10);
        }
    }
}

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class fireProjectile : MonoBehaviour
{
    // Drag & drop your prefabs in the inspector
    [SerializeField]
    private Rigidbody[] ammunitions;
    private int selectedAmmunitionIndex = -1;

    private Rigidbody SelectedAmmunition
    {
        get
        {
               return selectedAmmunitionIndex >= 0 &&
                   selectedAmmunitionIndex < ammunitions.Length
                   ? ammunitions[selectedAmmunitionIndex] : null;
        }
    }

    // Specify this function in the `onValueChanged` event
    // of your drop-down, in the inspector
    // Choose the function under the **dynamic** section
    public void SelectAmmunition ( int index )
    {
        selectedAmmunitionIndex = index;
    }

    void Update()
    {
          if (Input.GetButtonDown("Fire1") && SelectedAmmunition != null)
          {
              Rigidbody clone = Instantiate(SelectedAmmunition, transform.position, transform.rotation);
              clone.velocity = transform.forward * 10);
          }
    } 
}

Got it working.
Thanks so much @Hellium . That helps heaps!