In some states (I think still the majority), you can purchase a fully automatic M16 manufactured or imported before May 19, 1986 and registered with ATF. There is a ton of paperwork and since the supply is limited, it won't be cheap. And it will use a lot of ammo.
But he is nuts! The semi-automatic AR that looks a bit like an M4 is referred to by some as an "M4gery". It's just a semi-automatic carbine that is black and scary. I'd love to be able to buy a real M4, but I can't legally do so.
Not really important, since the main use of full automatic fire is burning up ammo at the end of the fiscal year.