Aluminium-cased ammunition is much cheaper. However, it adds 1 UNRELIABILITY POINT to whichever weapon is firing it. Furthermore, aluminium casings cannot be used to recycle ammunition or create handloads.
Australia, possessing the world's largest reserves of Bauxite ore by far, is a leading producer of aluminium. Since the main cost of bullets is the metal casing, cheap aluminium allows Oceanykan ammunition manufacturers to offer significantly cheaper prices for aluminium-cased rounds. However, these deform more easily than brass ones, cannot be reloaded and cannot sustain high pressures.