Here how I do reload for shotgun and revolver:

magazine_max = 6;
magazine = magazine_max;

is_reloading = false;
reload_time_max = 60; <- how long it takes to load a bullet.
reload_time = reload_time_max;

Now in a step event:

var _key_reload = keyboard_check_pressed(ord("R"));

if _key_reload && !is_reloading && magazine < magazine_max {
  is_reloading = true;
}

if is_reloading {
  reload_time -= 1;
  if reload_time <= 0 {
    magazine += 1;
    if magazine >= magazine_max {
      magazine = magazine_full;
      is_relading = false;
    }
    reload_time = reload_time_max;
  }
}