A shellcase body for use as part of an ammunition cartridge, which includes a base portion at one end, a middle portion having a substantially straight sidewall and joined with the base portion, and a shoulder portion joined to and extending from the middle portion. A neck portion may be joined to and extend from the shoulder portion. The shoulder portion is typically annularly shaped and includes a semi-circular sidewall that extends between an aft end and a fore end. The semi-circular sidewall has a curvature that is defined by a circular arc having a predetermined radius and a center that is positioned a distance away from the shellcase body center longitudinal axis. The shoulder portion is joined with the straight sidewall at a secant point of the circular arc, i.e., the straight sidewall defines a secant line that intersects the circular arc at the aforementioned secant point.