Development resumed in the late 1930s when the Panzerbüchse 38 came around after the need to provide infantry with a man-portable anti tank weapon.
The round originally had a steel core and a tiny capsule of tear gas.
The round was to penetrate the armor of the tank and the tear gas to force out the occupants of the vehicle.
The idea was impractical due to the core penetrating the armor and leaving the capsule outside.
SmK means "Spitzgeschoss mit Kern", which translates to "pointed bullet with core".