If you’re a Class 3 FFL, you still need permission from local PD chief as they’re technically for demo use only (like a PD wants to test before buying).
Individuals cannot. They need to pay for the $200 tax stamp and buy a gun that was made transferable before 1986. So it leaves anything new off the table. They did make “lightning links” which convert and AR to full auto, but they’re still expensive and were made before 1986 and be transferable.
Transferable M16s are usually $20k+, same with anything HK.