Introduction: Bullet Shell Valve Stem Caps
The inner diameter of a 40 cal. bullet shell happens to fit nicely over a valve stem cap. All you have to do tap the shell onto the cap with a hammer. These went to the same friend who got the retractable bullet pen.
Safety first: The shells were already expended by the time I got them. Never attempt to use a live round for any kind of project. Serious injury or death can occur.