Aside from a small handful of very old M-16’s or smuggled/illegally modified rifles, all you can legally get in AR rifles is semi-automatic. One trigger pull, one shot. You can get a “bump firing” type of stock or device that will cause the recoil motion of the rifle to make your finger cycle the trigger automatically. It can also be done (with practice) by holding the rifle a certain way.
Here’s a home-made “bump board” for a semi-auto pistol: https://www.youtube.com/watch?v=nlFZOBiWRko
Fill out the proper paperwork, pay $200 (per device), wait for the feds to get done crawling through your bowels with a magnifying glass, pick up your fully automatic firearm, short barreled rifle or short barreled shot gun. Just make sure you keep that paperwork with the item at all times. People with full autos and SBRs are the most highly vetted individuals on the planet. Way more vetted than the current usurper in chief.
What is this obsession with “full automatic” weapons?
Very few people can hit anything while firing full auto, unless the target is close. You mainly are just throwing lead downrange to get the enemy to keep his head down.
Anyone ever hear of “hold ‘em & squeeze ‘em”?
If you are trying to hit your target, full auto is about as useful as the TV/Movie gangbangers holding their pistols horizontally.