Process for producing a shell which extends about a longitudinal axis, as a component for a gastight packaging, for example a food can, which shell, in the vicinity of an end, as seen in cross section, has a first contour which comprises N (N >/= 3) inwardly concave rim contour sections, between which there are N substantially straight rim contour sections, and which shell, for the area between the ends, as seen in cross section, has a second contour which comprises N straight shell contour sections, which correspond to the straight rim contour sections and, as seen in the peripheral direction, adjoin a neighbouring straight shell contour section via, in succession, an inwardly concave shell contour section, a corner panel contour section and an inwardly concave shell contour section. The process is characterized by the conditions which the applied stretching and the ratio of the applied stretching in the first and second contours have to satisfy in order to produce a shell from material which is as thin as possible without the material fracturing or, in particular in the transition zone from the rounded cross section to the flattened, rounded cross section, without the material wrinkling, becoming dented or being pulled convex or concave in some other way. The present invention also relates to the production of a necked metal packaging container, characterized in that the neck shape is imparted to a starting shell before the N-cornered shape is imparted. The invention also provides an expansion tool which is suitable for carrying out the process.