I mean, for example, how difficult is it to legally obtain a modern style full auto AR-15 without a class 3 ffl? something made since, say 2005 rather than a vietnam era M-16 lol. (which would be cool too though)
If you have the money, this is easy to do. With AR's, either the Lower receiver is registered; the Drop In Auto Sear is registered, or a Lightning Link is registered. With all three of these, it is possible to configure a modern era M16 using a variety of barreled upper receivers or host guns.
Don't expect to be able to get into an M16 for under $10,000 - which is about what registered lightning links are going for (last I checked). DIAS's and RR's are significantly more money.