A cup and plug assembly of a base for a polymer based ammunition cartridge case body includes a cylindrical sleeve joined to the base. The base includes a plug molded over a cup. The cup includes a first annular cup portion extending in a first direction from a web formed therein and a second annular cup portion extending in a second direction from the web opposite to the first direction. The plug having a first annular portion that completely encompasses an outer circumferential surface of the cup and a second annular portion that extends from the first annular portion. The second annular portion of the plug completely covering an inner surface of the first annular cup portion of the cup. The cup preferably being formed from a metal or a suitable non-metal and the plug being formed from a suitable polymer that is injection molded over the cup.