I really suggest you don't do it, something really bad could happen, if you give a spudgun the oppertunity to hurt someone, chances are it will.
i didn't see how a snowball could be any more dangerous than a paintball
Well a snowball is roughly 20x bigger, travelling slower but the rules state that you can only discuss it if it's fired from a legal paintball gun, i.e one you buy from a shop, it can't be homemade. Some homemade paintballs guns have managed to fire paintballs over 700fps.
So firing snow isn't safe at all, for the reason of brittle PVC in the cold and how much it could hurt someone else, i'm not sure what the rules are over there in America but you probably could have it taken off you if caught by a cop.
Also if something does go bad, then you'll just give us more bad press showing how 'dangerous' spudguns are.