They'll always remain on top of roofs, although the Elite rangers will often jetpack between roofs.
It's the most beneficial way of reloading the monster 50 BMG. https://baidubookmark.com/story17880000/10-%D9%86%DA%A9%D8%AA%D9%87-%DA%A9%D9%87-%D8%A8%D8%B1%D8%A7%DB%8C-%D8%B3%D8%A7%D8%AE%D8%AA-%D9%BE%D8%A7%D9%88%D8%B1%D9%BE%D9%88%DB%8C%D9%86%D8%AA-%D8%AD%D8%B1%D9%81%D9%87%D8%A7%DB%8C-%D8%A8%D8%A7%DB%8C%D8%AF