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Technically, thieves are allowed to carry crossbows and short bows. However, most of the magic crossbows and short bows in the game say in their description that they are "not usable" by thieves. I think a +1 short bow is the best that is usable.
So in terms of allowable weapons, exactly how viable is a ranged thief (or say, thief/mage) if you don't want to dual/multi it with a fighter class?