Is this true?
I don't want to upgrade my heavy crossbow if it is true because it's the only one in the game. I did test a light crossbow and it *does* deal lightning damage.
A lightning light crossbow +0 does 48 or 52 damage on knights in Anor Londo using wood bolts or standard bolts, respectively. With the lightning light crossbow firing a lightning bolt the lightning effect is visible and it does 75 damage. By comparison, my heavy crossbow +10 does 96 damage with a wood bolt.
Ideally I'd like to upgrade the heavy crossbow to lightning +5 but not if it doesn't work the same as the light crossbow.