the correct answer depends on both your state and current federal laws,
in most states,
you must purchase a machine-gun registered prior to 1986
and pay a $200 tax after a back ground check to own a legal machine-gun
since there's only something like 120 thousand legal guns registered prices are absurdly high.
there are some other minor restrictions like you must keep the paperwork accessible and you can't travel out of state without prior written permission. (and you must store it in a safe)
the paper works a P.I.T.A. and the wait times are excessive, and the cost is prohibitive, all on purpose, but it's potentially and legally possible, in many states. yeah if your really into firearms... a fun toy and a lot of red tape! https://www.atf.gov/firearms/applications-eforms-support...
garage.grumpysperformance.com
BTW I know a few people that own legal fully auto, guns, most gun ranges won't allow them, even with paperwork showing they are legal
the feds can make it a P.I.T.A. to own one legally, just because they can.(different states and ATF offices have wildly different attitudes)
btw the similar conditions exist for suppressors in that
in most states,
you must purchase a suppressor if you buy a new or registered existing one and pay a $200 tax
recently the ATF made the process much faster and much easier, a vendor familiar with selling legal suppressors will walk you through fast and easily