Abstract: A multi-piece ammunition cartridge casing includes a casing base extending along a central axis and defining an opening extending axially into the casing base from the distal base end. The inner casing surface defines a circumferential groove with a groove radius greater than a radius of an adjacent portion of the opening. The cartridge casing also includes a casing body with a proximal end portion disposed in the opening of the casing base. The casing body is secured to the casing base by way of a flange protruding radially outward in mating contact with the circumferential groove. A flash opening extends from the primer pocket to the inner face, where at least part of the flash opening expands in size moving toward the inner face. An ammunition cartridge incorporating the multi-piece cartridge casing and methods of making are also disclosed.