In the 20 use 4s or 6s. Even 6s should penatrate a mallard sufficiently at 35-40 yards. The extra pellets will also help offset using a 20 somewhat. You could also try 3s, but, You shouldn't be using a 20 much past 35-40 yards and that is when the 3s would really be needed. 2s are just going to have too many holes in the pattern when they get out to the range where they are needed.