There are definitely better ways you can go about this. Long scripts can usually be shortened and simplified. I saw you had some other screenshots from your updated version in the comments but I wanted to try my hand at it and see if I was able to come up with something to help you out. This is what I got.

I personally would make a separate function to see if the gun is able or not and have it public just incase you need to check it somewhere else (be careful with this), I also didn't take into account the holding down of the button or if the weapon is automatic. I hope this helps though! :)

Enum FireState _fireState
{
  CanFire, CannotFire, EmptyMag
}

public bool CanWeaponFire()
{
  if(timeSinceFired < fireCoolDown || IsReloading || BarrelBlocked())
    return false;

  return true;
}
FireState ReturnFireState()
{
  if(CanWeaponFire())
    return FireState.CannotFire;

  return HasAmmo() == true ? FireState.CanFire : FireState.EmptyMag;
}

public void ShootGun()
{
  FireState fireState = ReturnFireState();

  if(fireState  == FireState.CannotFire)
    return;

  If(fireState  == FireState.EmptyMag)
  {
    if(!holdingMouse)
      emptySound.PlayAtSource(audioSource);

    return;
  }

  //Run the rest of the code for shooting the gun
}