A forearm for a firearm that includes a magnesium alloy body. The forearm can be adapted for use in a modular rifle having both an upper receiver securable to a barrel and a lower receiver housing a trigger assembly. The magnesium body may also be nickel plated and have a ceramic coating on the nickel plating. Also disclosed are methods of manufacturing a forearm having a magnesium alloy body. The magnesium alloy body may be formed by extrusion or by thixotropic molding. In some embodiments, the magnesium forearm includes an outer sleeve formed out of a magnesium alloy and an inner sleeve formed out of a different material.