Hi
I did small experiment since I have a volcano hot end and it's veery difficult to manage a not oversized tip.
As I read, there is a smaller dimension tube to link direct extruder with hot end (like 1,8mm internal) shipped with sets we had available to order. No such sets for now, shopping is difficult since even if the dimension declared is like needed (ID 1,8mm OD 4,0mm) it's reported by buyers not always be true.
I decided to do my own shaping tube in a simple way. Just use a lighter, burn a tube (capricorn in my case) for about 10 sec and pull. The tube is extending the length and decreasing dimension locally. Pulling by fingers was enough, still some force was necessary.
Enclosed pictures of results.
It works like a charm, my 3dchameleon started to work reliably.
Please remember to not breathe the fumes and use good ventilation doing this trick. Overheated ptfe tube produce some nasty chemicals, as I heard.
Picture shows modified Capricorn tube. Not the filament.