Hi Alpha,
As a full bore rifle shooter and previously pistol shooter I amassed a collection of reloading implements and indeed as I have friends who are RFDs....
The swageing I am referring to is where the brass fitting on the coupler is too big by a couple of thou, you could just turn this off as brass is soft(ish) but as I have an expanding neck die I just open it up a fraction( to get a friction fit) insert the threaded coupler and seat crimp as I would a bullet.
Alternatively you could turn a 12mm dowel fit the coupler and glue this into the cases...
The rollerball is a sloppy fit already so I will be taking the above approach and using an appropriate sized brass washer on the collar. It needs something internal to support the refill and spring so seems the most logical approach....
Watch this space