Apparently Mr. Heller attempted to register what DC considers a "Machine Gun."
Despite all the hand-wringing here and on THR about the "machine gun" designation, I think that's a red herring. The District has chosen to define guns that meet a certain set of characteristics to be in a category that they have chosen to call "machine guns." Governmental bodies do this all the time--creating definitions for words that have different meanings within the scope of their particular law than what they mean in common English. It would make no difference to anything if they had chosen to call that category "ham sandwiches." "Mr. Heller, you cannot register that gun because it is a 'ham sandwich'." It's the same difference.
The real issue at hand is that the Supreme Court held that they can't ban
any class of commonly-used firearm. (At least I think that was "holding" and not "
dicta.") The M1911A1 is the quintessential "commonly-used firearm"; it matters not whether you call it a "semi-automatic pistol," a "ham sandwich," or a "machine gun." As such, it is clearly in the range of weapons that the USSC had in mind when it ordered the District to register Mr. Heller's gun.