I was tinkering today with a couple of 15's to figure out what the issue is with the loose cap they seem to be known for.
As I was screwing the thing, I noticed that if I didn't screw the section all the way into the barrel the cap would snap nicely.
This is a pen for which I tried the hot water trick (filling the cap 2 or 3 times with boiling water and leaving for 30 seconds to nudge the inner cap's plastic back into shape). While that helped quite a bit, it was still too loose to trust that if I clip the pen it won't fall off.
So I took a piece of 22 ga silver wire (22 Gauge, 1/10 sterling silver filled wire purchased from Amazon, to be exact) and made me a spacer ring.
The cap snaps nicely in place and it looks well enough not to be a noticeable hack.