Hi Ed,
The best pellets I’ve ever used with the HW40 were some JSB Match Diablo Light Weight in 4.48mm head size.
Completely by chance these pellets came with a HW40 purchase and they were superb so I’m assuming the previous owner had done his homework or struck lucky.
I’ve usually always preferred the usual bigger stated head sizes on pellets but that’s usually with rifles and was a tad dubious with the 4.48mm but they really were absolutely superb.
I did try to find some to order but could only find 4.49mm at the time and I think from our sponsors JSR but that pistol and pellets are long gone now.
Nowadays I would just use the usual RWS R10 Match or Hobbies etc