How can a person legally obtain NFA firearms?
A person may make an NFA firearm by filing and receiving an approved ATF Form 1 Application to Make and Register a Firearm.
A person may transfer an NFA firearm to another person by filing and receiving an approved ATF Form 4, Application for Tax Paid Transfer and Registration of Firearm.
Applications to make or transfer a firearm will not be approved if Federal, State, or local law prohibits the making or possession of the firearm.
[26 U.S.C. 5812 and 5822; 27 CFR 479.62 and 479.84]
Source ATF: https://www.atf.gov/firearms/qa/how-can-person-legally-obtain-nfa-firearms