If you jam these onto the cap as far as you possibly can, then they will
probably stay on for a fairly long time, but if you trust them to hold a bullet
Space Pen in your shirt pocket, eventually you'll be disappointed. I've had
years of experience with these, and sooner or later they come off when you least
expect it. I keep a bullet Space Pen in a pants pocket, and this clip is mainly
useful for keeping the pen from rolling off a table if I put it down there.