Wonderful calibre. I have a HW95 and HW80 in .20 and they both perform really well.
As for gas rams I agree with Matt. The .20 Fenman or Evolutions are great rifle in so many ways. As for the mighty FAC Eliminator in .20-this superb gun is an experience not to be missed once you get used to the firing cycle.
Pellets-OK not as many varieties as in .177 or .22 but I have found the FAC .20 great with JSB and Crosman Premier. The 12 ft lb rifles are usually OK with FTT or JSB which although heavier are still very well made widely available pellets.