You'll have to get a pre-ban M16 or pre-ban parts that were registered prior to 1986. The cost of the tax stamp (permit) will be a drop in the bucket compared to the $10,000 to $20,000 the rifle/parts will cost you.
If you are serious the first thing that you need to do is visit your chief of law enforcement (county sheriff/local police chief) and see if they are willing to sign off on BATF forms for you to have the weapon.
Next you will have to locate a class III dealer in your area who will handle the transfer for you. Not all FFL holders have a class III license.
Step 3 is to locate the weapon and purchase(pay for the weapon). once paid for you have to wait for the ATF to authorize transfer from selling dealer to local dealer who will hold weapon and process transfer to you. Takes 2-8 weeks.
step 4. Once you local dealer receives the weapon you will have to have your ATF paperwork filled out and signed by head LEO (police chief/sheriff). Procure 2 passport photo's, get finger printed on two of the cards provided by dealer, and write a check for $200 to the BATF.
Once all of this is complete your dealer sends it in to the BATF where an agent will be assigned to your transfer, they do god knows what while taking about 6 months to approve transfer.
Dealer recives your tax stamp in the mail and then you can take your full auto home.