Machine guns are largely banned, except for some pre-ban guns that are heavily regulated and registered. Now, when's the last time anyone here read a story where a bad guy used a fully-auto machine gun to commit a crime or a massacre?
So the mantra of "if guns are outlawed, only outlaws will have guns" would seem to be disproven using that example, now wouldn't it? We banned machine guns...and criminals no longer had a supply of machine guns in civilian hands to draw from (either through theft or straw purchase). Fancy that.