NFA Forms
 

I'm posting here direct links to the necessary forms for acquiring Title II weapons (NFA items) in the United States.

If you buy through a licensed dealer, they'll certainly have the appropriate form for you. But if you choose to build your own, you'll need Form 1. Or if you inherit one, you'll need Form 5. Or if you buy from another resident of your state you may not need to go through a dealer, and Form 4 would be what you need.

Application to Make and Register a Firearm (Form 1)

Application for Tax Paid Transfer and Registration of Firearm (Form 4)

Application for Tax Exempt Transfer and Registration of Firearm (Form 5)

Certification of Compliance with 18 U.S.C. 922(g)(5)(B) A.K.A. Certification of Citizenship