Well the 15mm caps are made with a recess in them for the flange and yes it will help in making sure you dont slide it too far.
When you put the caps into these tubes you need to be careful in keeping the cap square to the tube not angled like in the picture, there is a point there where the metal can touch the glass until the oring is fully into the tube and if you put pressure on it while the metal is on the glass you can break the tube.
The 15s are most certainly more delicate than the other sizes we have and I would say its best not to remove the caps unless you need to because installing them is probably the biggest chance or breaking them short of a catastrophic impact. If you would like to send it back to us we would be happy to install them and return it. I could also send you a smaller oring that would not fit as tight, we settled on this one because we were able to assemble them fine, into the thousands now with very few issues but then we put together a lot of
tanks.
The way I put them in is first off of course to lube them with
eliquid then give them a bit of a press and twist action to get it started, same with the other end then kind of a pumping action with thumb and forefinger on the caps to make sure they are all the way in.
Just let me know how you want to handle it and we will fix you up.
Also, remember the back up the cap that wants to come out when sliding the cartomizer and you will never have any trouble with a cap coming out on you while filling, if you are holding the right cap then everything is trying to push together rather than apart.
