I'm setting up my 20 and have been happiest with hevi13 size 7 shot. Since the 7's have plenty of energy to my maximum range, I don't see a reason to use space in the hull with the 5's & 6's. I want the highest pellet count possible. I'm using a Primos tightwad @ 570 and a Carlson @ 575 and don't see much difference between them.