The X-Bow would probably have to be pointed at an angle greater than 45 deg. in order to hit air targets accurately, especially at short range. Therefore darts could possibly be shot in a skinnier parabola-shape fashion similar to a mortar shot, which would reduce the x-bow's range. If it looks like it's pointed up at 45 degrees, or if the darts don't seem to travel in such a way, I kindly remind you that there's more than one graphical glitch involving x-bows at this time.
Edit: I apologize for the formal language, I don't exactly know how to convey this w/o getting technical.
