Provided is an increased capacity firearm ammunition magazine with a metallic body. The body is dimensioned to compatibly replace a (OEM) polymer or polymer over metal single stack magazine. The body has walls with exterior dimensions compatibly matching those of the OEM magazine and that are thinner than those of the OEM magazine. The thinner walls allow an at least partially laterally offset arrangement of cartridges therein. A longitudinal exterior ridge extends along a forward wall of the body and is configured from the metallic body walls and sized to compensate for forward-to-aft dimensioning difference compared to that of the OEM magazine.