Prior to 1986, a person could have paid the $200.00 tax and then convert their AR-15 to fire full auto.
You would have a Colt marked AR-15 with M-16 internals. Many people did this.
I have seen one AR-15 marked Colt that was full auto. The owner stated it was not a conversion job, but a factory direct Colt.
So yes, there are Colt marked AR-15 machine guns that are fully transferable out there. They start out in price at around $13,000.00 and go up from there.