An ammunition case reloading die assembly, and method of use, that provides calibrated adjustment of taper imposed on the mouth of an ammunition case. A calibrating ring or cap bears indicia in the form of polygonal faces, splines or radially-directed markings. The ring or cap rotates with an upper portion of the die for threaded adjustment of the position of the die within the frame of a reloading press. The die has a longitudinal bore, a lower portion whereof is tapered for imposing a tapered crimp upon the mouth of the case. The taper of the longitudinal bore is preferably chosen to make each partial rotation of the calibrating ring or cap through one indicium correspond to increasing the taper imposed on the case mouth by one thousandth of an inch per case longitudinal inch.