In my experience with HW .22 barrels you may find Webley Mosquito Express or Falcon Accuracy Plus worth a try. They're only available in 5.52mm.
H&N FTT's are also worth comparing again start with 5.52 but other sizes are available. Weihrauch FT pellets are very similar if not identical, I'm pretty certain they are re-badged H&N FTT's.
I also used to get good groups with Defiant 5.52mm, haven't yet tried the H&N version but they may well be good.