You "store" a missile attack spell that you can cast and are proficient in in the target weapon. For the duration of Spell Store, you gain Limited Improved Spell Strike. Only one spell can be stored in the target weapon at a time.
Limitation: Can only be used with the stored spell and the item it is stored in.
Improvement: You are considered to have and be proficient in the stored spell for this purpose.
Target takes the "on hit" effect of a conjured missile spell that you are proficient in. If the triggering hit was a critical hit, then the target takes the "on critical hit" effect instead.