The end result would be that responsible gun owners who wished to practice at a shooting range could no longer afford to, and criminals would continue uninterrupted because they would just steal the ammo they need.

A variation on this theme already happens with cigarettes, when one state raises tobacco taxes, bootlegging from neighbouring states goes up.

Now the responsible gun owners are no longer a very good shot, criminals are roaming around with plenty of ammo (which they need because they often have terrible aim), what could possibly go wrong?