Hi, thank you for answering. Yes i am pretty sure i followed correctly the steps in the link, i am attaching the image of the geometry and of the named selections:
in red I highlighted the fins (name selection: fins) in which I later use Proximity, Scope To: edges while in blue I highlighted the boat-tail (name selection: boat) for which I use Proximity, Scope To: faces and edges.
Here is an image of the bullet enclosure and the named selections:
tell me if you need anything else.