Why not use lesser missile storm and greater missile storm?

As for the magic bullets: You could make it: Swift action fire, once per round, one round per level. Ranged touch. In this case, you can, at 11th level, shoot four bullets, each one dealing 1 damage, okay. But you could use the rest of your turn to cast a spell, run away, slash something, or.. Well, you could use the rest of your turn to do something useful.
However, perhaps it's better to make it a second level spell rather than a fifth.. Just if you look at the impact it will not have during a combat.

Or a repetitive magic missile spell that does work on concentration.. I like that option.