I've not had an interest in reading every post in the thread, but I see no one has attended to this question, so I'll give it a whack.
Yes, there is a extra short carto tank available from IBTanked called a "
Halfling" tank. It uses a special made, short (20mm vs a standard 35mm or 45mm XL carto) 2 hole, single coil Smok brand carto in 2.0 or 3.2 ohms. They are (currently) available in the
22mm diameter size only and hold approximately 1.2ml... not counting what's in the carto itself, so closer to 1.8ml. You would have to ask IBT if they will be doing a 19 or 25 mm diameter Halfling again.
On an 18350 mech. As you can see, with a stubby drip tip, well under 40mm total.