I know for a fact that a 1/8 - 1/9 will hammer the 140's and 142's. The 1/8 seems to be the norm for most shooting the heavier pills, mostly in 24-26" barrels.
I wanted to try a 1/6.5 in a 20" but I think I will start with a 1/7 in a 22" since my JP gasser is 22". I can keep similar ballistics...