Not impossible. No need to ban, simply regulate and tax.
Every change to the system has downstream effects. Sometimes people don't notice a bit of indirection (which is why it is a good defensive weapon). Regulate lead, for "environmental reasons", and put a heavy tax on it "to pay for environmental projects", and bullets could soar in price without even having to ban them outright. (You could even have Obama give a passionate "crocodile tears" speech about the right to self-defense, just to make sure that 99% of the population failed to make the connection!).
This would cause them to become more unaffordable, and the total number of retail outlets carrying them to decline. It would create a self-reinforcing spiral, to reduce the total amount of guns and ammo in the system.