The definition of "Hülse" is as follows:(Patrone) case, (Granate, Schrotpatrone) shell, (Gewehr, Pistole) receiver (MG) gas cylinder plug, (Gewinde) socket.

However, I have seen descriptions of manufacturing procedures which resemble what you are describing. It is as if the term "Hülse" was being used to describe what I would translate as a mandrel, such as a mandrel used in hammer forged barrel manufacture, except that rather than being a tool of the manufacturing process it constitutes an integral part of the finished product.