A reference post because I wish I had been able to find one.
I have a Hatsan Junior 20G. If you are considering one of these you might like to know it shoots 21g plastic wad cartridges beautifully. It does not however shoot 21g fibre wad cartridges.
Thanks to the awesome Mr @TIGHTCHOKE for sorting that one out for me.
For the search engine: IMW A Hatsan Junior semi-automatic will not shoot 21g fibre wad cartridges.
If anyone has evidence to the contrary please do post. The Hull fibre pro-ones are jolly good clay cartridges but there may be other brands that work in the semi-auto. Never say never.