Abstract: A firearm cartridge case capable of withstanding internal gas pressures of at least about 50,000 psi has two substantially cylindrical portions of significantly different diameters interconnected by a frusto-conical shoulder portion. The case has a ratio of its overall length to its diameter, at a location 1.25 inches from its base, of no more than about 4.2, giving it an unusually short, fat profile. The base of the case has a rim with an outer diameter substantially no less than the case diameter at the aforementioned location, to facilitate reliable feeding of the cartridge from the magazine. The corresponding firearm chamber which matingly accepts the cartridge is capable of withstanding internal gas pressures of at least about 65,000 psi.