Basically with barrels and paint rounds you want a boundry layer of air all around the ball, nothing too big. But nothing to small that it will rub on the barrel. FS rounds typically are .685" to .687" (most of mine are .685 after measuring them all)
underboaring the barrel will cause a loss of accuracy and range as the ball will be "forced" through the barrel.
Overboaring will cause an extreme loss in accuracy as the round will "tumble" through the barrel and the trajectory as it leaves the barrel is totally unknowable.
Personally id say keep around .685. To me thats the best performance size
Legion LV1 (Red/Black) - Stock, who really needs an upgrade. Rotor, Ninja 68/4500 Grey Ghost
Akimbo Tipx FS - FS blocks, PC3 dual remote, Zetas