The absolute easiest way is for someone to
get a Federal Firearms License or “FFL” (even a
home-based FFL).
Depending on the
type of FFL, the FFL-holder can purchase and sell machine guns, regardless of when they were made (more on this below), and they can even legally make their own machine guns or lawfully convert current firearms into full-autos. The best part about getting a machine gun as an FFL is that you can get it at dealer cost and fast.
Even without an FFL, a private citizen can still lawfully own a true machine gun if certain conditions are met. However, machine guns for non-FFLs are EXTREMELY expensive as the available supply is limited.
As an FFL, you can buy a brand new machine gun for less than $2,000 and have it transferred to you in a few days.
As a private citizen (without an FFL) you can only buy an old machine gun (over 35 years old), it’ll likely cost north of $15,000, and you’ll have to wait around a year for the transfer via an
ATF Form 4.