.20 It's a lot easier to shoot this calibre than it is to talk about Map out the pellets fight path with .20 N&N FTT and you'll find, If you've shot .177 bis mags there really isn't much in it between the two them.

A skill of the air gun hunter is to carry a pellet suitable for close range shoots, .177 tend to drill through .20 is less likely too and if JSB .20 pellets are to tight in your barrel, sizing them will give flat surface to the head of the pellet, which can be useful and will help stabilise the pellet at range.