BATFE Rules
The National Firearms (NFA) Act of 1934 required the registration and taxation of weapons such as machineguns, silencers, short barrel rifles and shotguns, destructive devices and AOW.
Find more information here direct from BATFE site:
http://www.cs.cmu.edu/afs/cs.cmu.edu/user/wbardwel/public/nfalist/nfa_faq.txt
Valuable information can be found also here:
https://www.atf.gov/firearms/statelaws/26thedition/index.htm
http://www.cs.cmu.edu/afs/cs.cmu.edu/user/wbardwel/public/nfalist/semi_auto_faq.txt
http://www.nraila.org/GunLaws/Default.aspx