Multi-piece cartridge casing and method of making

An ammunition cartridge casing comprising:

• a casing base of a first material, wherein:
• the casing base extends along a central axis from a casing head to a distal base end;
• the casing base defines a primer cavity extending axially into the casing base from the casing head;
• the casing base has a generally cylindrical outer surface with an outer case diameter and an inner casing surface defining a body opening extending axially into the casing base from the distal base end to an inner face, wherein the body opening is located distally of the primer cavity and includes a first portion of a first inner diameter extending axially;
• the body opening further defines a circumferential groove located axially between the first portion and the inner face, the circumferential groove having a second inner diameter greater than the first inner diameter of the first portion;
• and
• the casing base further defining a flash opening between the primer cavity and the body opening, the flash opening expanding in size as the flash opening extends to the inner face;
• and
• a casing body of a second material, the casing body having a tubular sleeve portion extending away from the casing base along the central axis, the casing body also having a proximal body portion of reduced diameter received in the body opening and including a flange occupying the circumferential groove.