There are a few weapons that do not give skill shot xp when used in one. This is most notable in rebound, as these weapons will never deal damage. These weapons include napalm, heavy napalm, partition, and division. There could be more, but these are the only ones I can remember right now. Also, when these shots get a x2 and then hit, they deal x2 damage but do not give the 5 bonus xp for the skill shot.