...ah, hold on. This is because it's supposed to be possible to progress through the planet without importing copper, right? The rocket launcher recipe has electronic circuits in it.
Still, you could add an alternative recipe for the rocket launcher, or a new, worse rocket launcher that only uses iron. Or make it cannon ammo so the galleon can fire it! It's just weird for it to be a bullet, and awkward to use since the submachine gun fires so rapidly.