[MultipleParadox]

.25 or .28 are great in those pistols, even at relatively low fps. I usually use 0.20 or 0.25 indoors and 0.28 outdoors

You'll gain in accuracy regardless of fps; but your BB will fly slower, that's all

Also, as mentioned, you'll benefit from Joules creep

A gun shooting 0.2g BBs at say, 320fps, will output 0.95J

With the same gun, if you just swap to 0.28g, you might find out that yes your velocity is lower (made up number, say 300fps), it adds up to more energy; that would be 1.16J

The danger is that if you rig your pistol to shoot 350fps with 0.2g (1.13J, that would be most indoor fields limit), and you swap to 0.28g, you'll shoot too hot and could be refused at the door