Note the reason why I run .25's indoors is because of the shorter engagement distances and the heavier the BB the longer it stays in the barrel for gas to act on it (gas expansion vs a more "linear" compression of a piston in an electric). For a short pistol it's not a big difference but in a gas rifle with a longer barrel you can actually increase the energy output of the BB significantly when using heavier BB's.
tl;dr Mind the energy output of a gas gun and the weight of BB you use. While in an electric it's more or less linear so a .20 will take the same energy as a .25 or heavier (they just move slower due to the momentum equation "m1v1=m2v2").
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