A casing for use in a cartridge for a firearm comprises a sleeve and attached base. The sleeve has cylindrical portion with a mouth for holding a bullet and an opposing radially-running bulkhead from which extends a nipple that is secured within a passageway of the base by a lip at the nipple terminal end. There are one or more seal regions in the passageway. A sleeve is preferably made of austenitic stainless steel and there are differing hardness and magnetic properties along the sleeve length. Particularly for high pressure rounds, the bulkhead has a frusto-conical portion with one or more circumferential waves and mates with a conical depression in the surface of the base; optionally, a skirt around the periphery of the base provides support to the sleeve bulkhead. A base has a deep cannelure with a continuously curving groove bottom, enabling a base to be made of an iron alloy into which carbon is diffused to enable hardening, after machining.