A method and apparatus for sizing ammunition cartridge cases includes a cartridge case trimmer including a cylindrical housing, a mandrel detachably coupled to a longitudinally-oriented opening formed in a first side of the cylindrical housing, the mandrel having a first end including a cutting tool disposed in a slot on the first end and comprising one or more notched cutting surfaces, the mandrel having a second end opposing the first end, the second end protruding from the first side of the cylindrical housing and sized to be received in a chuck, a bearing disposed within the cylindrical housing adjacent to a second side of the cylindrical housing, and an insert comprising a tapered region disposed in an inside diameter of an inner ring of the bearing.