If you buy from a dealer they will walk you through the paper work. You have to decide if you want to register it to a trust, corp or personal.

The only difference between an ar and m16 is the fire control parts and the bolt carrier. The cheapest m16 you can get is going to be a sendra conversion they run about 13K.

Edit to add: Link to a Larry Bardwell explanation.

http://www.cs.cmu.edu/afs/cs/usr/wba...st/nfa_faq.txt