Disclosed is an ammunition cartridge case that includes a sleeve, a base fixedly attached to the sleeve and a fiber reinforced polymer composite annulus that is at least partially within the base. In certain embodiments, the sleeve and the base are formed partly or entirely from a metal, for example steel or stainless steel. The base has a central aperture that affords for the annulus ring to fit within. The annulus is dimensioned such that it fits securely within the central aperture of the base and has a center aperture wherein a primer can be located. The annulus ring contains a volume of composite fibers ranging from 10 to 90 volume percent. The annulus isolates the primer from the cartridge case and thereby prevents galvanic corrosion between the primer and case. In addition, a metallic foil can be present between the annulus and the cartridge case in order to assure proper securement of the annulus therewithin.