If the aim is to shoot, we can stick to rubber bands. I think the aim here is to have a workable gun: accurate within reasonable limits (say, 75-100 meters), and lethal, or the very least, damaging. A tumbling bullet is not accurate, and likely slows down enough to be non-lethal even.
If these criteria are not met, I see absolutely no point in a printable gun. If they are, they can be a great tool for national defense militias: a network of such printers could churn out these low-grade weapons quickly and cheaply, arming militias and citizens to kill their attackers and likely take their weapon. In a sense, it could be an evolved, more usable version of the Liberator, that can potentially double as a backup weapon even after its initial use.