Anyone like flip up breech underlevers

[Geezer]

Not sure why you would think that - it is simply a barrel sliding within the fixed outer sleeve
any decent lathe could do a good job

remember there is no great force on the barrel as with a break barrel

You may be right.

I’d suggest the difference is that a break barrel (assuming good breech-bolting) will tend, if it does give issues, to give issues in only the vertical plane, and any POI movement will usually occur progressively over long use and is easily compensated for.

Whereas a slidey thing could misalign in any direction.

Could be wrong. Just the slidey thing feels novel and maybe not a great idea.

[accuracy lover]

I think that Hatsan (or whoever thought or invented this solution) had a very good idea.
The possible accuracy issues can be easily solved by controled quality of manufacture of the various parts - for example if this system was made by Feinwerkbau or Anschutz the tolerances between parts would be much- much better.
The system can be developed in a way that it could allow easy and fast barrel interchangeability (swap the 0.177" barrel with a 0.22" one or a 0.20" in minutes - without the neccesity of special tools).
It could also be made in a free floating design allowing maximum potential in accuracy.
A clever designer could also add a simple mechanism for adjusting the point of impact on target so the shooter can make scope zeroing much easier (by altering the angle of the barrel inside the sleeve).
Just some food for thought...

[Geezer]

if this system was made by Feinwerkbau or Anschutz

Yes, maybe, but it’s made by Hatsan.

And I still wait to be persuaded that it’s a good idea, even if a combination of Patek Phillipe, Porsche and NASA were making it.