The point is that an overly large charge of powder will not enlarge, as they say on TV "that special part" of a man's body. So therefore there is not need to subject your self to it. Most of the CAS shooters I know shoot between 22 and 28 grains in their replica guns.
simonkenton, I recommend vtigunparts.com for all your replica and original gun part needs. They are specialists and will have most everything you will need.
I buy my TRESO nipples from http://www.possibleshop.com/