Always thought the 5mm calibre was a shameful way of Sheridan making profits on the dustbin pellets they sold to match their odd-calibre rifle.
Jack of all trades, master of none, the .177 and .22 are better for specific purposes, while the .20 does nothing well.
Daystate used to make a sizer to take a .22 pellet down to .20, that could be a way of widening choice, althout most pellets would be a bit on the heavy side.