A casing for use in a cartridge for a firearm comprises a sleeve and attached base. The sleeve is formed with a mouth for holding a bullet and an opposing bulkhead from which extends a nipple. The end of the nipple is flared radially outwardly within a passageway of the base, to form a special configuration lip and first seal. The nipple is shaped to make a second seal when press fitted into the passageway. A bulkhead is formed with a circumferential wave or ridge. A sleeve is preferably made from austenitic stainless steel that is worked to have differential hardness and magnetic properties along the sleeve length, with the nipple being of lesser hardness.