There is special licensing required. Not just any FFL can legally make a fully automatic firearm.
The ones that are licensed to do so can only retain the firearm as long as they are licensed, and they can only remain licensed as long as they are in the type of business that demonstrates the need for the licensing.
AND, any full auto made after 1986 will NEVER be legally transferable. Only law enforcement, military and properly licensed FFLs can own them, and only as long as their business/occupation justifies that ownership.
Here's a recent thread on the topic.