Turn the upper barrel to the shape you want, leave it a little larger than target size.
Turn the lower barrel down. Final size is fine.
Using a parting tool, remove about a quarter inch of wood from the upper barrel, all the way to the brass.
Find some wood you want to use as a CB. Cut an inch or shorter chunk of it with a 7mm hole in it.
Slide a 7mm tube into it and mill the end. Remove the tube.
Take the upper end off the mandrel. Give it a good coat of thick CA (thin may work, dunno).
Lay the CB, milled side up, on a bench and give it a shot of accellerator.
Press the upper barrel's brass tube into the CB. Hold down tight and even, dont let it set crooked.
After it cures, trim the end with a trimmer and reduce it's size to the goal size for the CB.
Mount everything back on the mandrel, turn the top end to size making sure to keep the transition of the CB nice and flat.
Give the edge of the CB a little bevel if you like. Try not to make the upper and lower diameters too close or too different. Either will make it look odd.
I've got a few in my photos.