Assuming you aren't a felon, you CAN purchase them. Full auto. Machine guns. The real deal. All it will cost you is a $200 tax stamp and (usually) a background check (which entails a delay of variable length). Well, and the cost of the gun itself.
Mind you, nobody (at all) can MAKE new ones for ownership by the general public in the US - but that isn't what you asked about. Because nobody can make new ones (this has been the case since 1986), the existing ones are getting more and more expensive - in fact, the $200 tax stamp is chump change compared to the cost of the gun itself - it's not uncommon to see cheap nasty full auto guns (that are in the $500 or less range w/o the full auto serial number) go for $5,000 or more.
Your state may limit things above and beyond this - but since you mentioned Second Amendment, I'm limiting my comment to the Federal situation.