I guess 24 15d6 fireballs in a single round will have to do :/
It just seems less fun though
Go read the ability for god's sake. That won't work either. I'm not going to tell you why. Do your own homework.
I must've glanced over the targeted part :/
EDIT: Now, I don't see why a spell like vampiric touch wouldn't work. The only thing is, would the hit points still be transferred to you even though you're not actually touching the person?
Yeah, that should work fine. It wouldn't be much different than applying the Reach Spell metamagic feat to it.
I'm not sure if the hit points would go to the caster, or the person who fired/threw the weapon, though. The Spell Storing description is not very clear on who "the caster" is. Some items explicitly say the person who uses them is (potions, for example).
It would be kind of crazy for a crafter to load up a bunch of Spell Storing weapons with Vampiric Touch before he sells them, and then gain temporary hit points at random times of the day for several weeks.