0.20 grams tell many people. In fact, hence our name, the more weight used in airsoft: 0.20 grams. But it is not the only one there.
Some airsofters prefer to use 0.23 or 0.25 balls to play or select rifleman shooter, especially when there is wind. And of course then there are the snipers who choose heavier balls: 0.30 g, 0.36 g or 0.40 g.
The use of balls as Guarder of 0.36 g do we need more power to achieve the same extent as with lighter (obviously) but we'll get that ball flying balls is our straight and firm.
As always there is no rigid to choose them manually, much depends on the players and their replicas. But if you decide by 0.36, try the Guarder, you have many possibilities that don't ever change.