If you happen to have a clear reciever as a spare due to a metal-body upgrade, I think you can slap that on as well as your stock orange-tipped/covered flash hider and shoot in your backyard without any legal issues, although I don't have any verification on that.
As far as being hit by a 0.25 instead of a 0.20, I've been hit by both at relatively the same FPS, and didn't really notice a difference as to the impact of it, bruises were the same, heh. Certainly didn't hurt more, or at least not enough to feel the difference drastically.
Primary: Custom WE M16A3
Secondary: KWA MP9
Side: Custom WE M1911A1