4mm M20 use??
 

Hi,

Does anyone here have any experience with the various 4mm conversion units that fire the M20 cartridge?

Yes. They work well :)

Since the 4mm conversions are single shot operating, I'm wondering how people feel about using them in a nicer Luger they may not otherwise wish to shoot with full power ammo...

They have the same feeling to them as air guns, so not too exciting.
But very practical for indoor training.