The ATFE does not really care how the military describes it, to them it is either an automatic weapon/machinegun or semiautomatic rifle, there really isn't anything in the middle.
The ATFE also uses a couple of criteria in deciding whether something can be 'Readily made into an automatic weapon,' mainly being the 8-hour rule, which is if their techs can get it to fire at least two rounds on one trigger squeeze after screwing with it for eight hours with the kind of tools to which you have access, to include duct-taping things in place (Yes, any repairs or mods do not have to last longer than popping the second round) then it's an automatic weapon. This means they will bust you if they simply catch you holding all the parts for a conversion, whether you've actually done anything toward it or not.
I don't know if there is anything valid here or not, with an AR even a polymer lower would work, the lower is the controlled 'Receiver' but it doesn't actually carry any stress from firing, the spring force from the fire control group is all it really needs to deal with. Although the pivot pins are placed slightly differently on a AR vs an M16/M4 (And there are adaptations for that), all the full-auto or select fire parts are in the lower, except the bolt carrier (And it is not difficult to either alter or just find an M16 carrier to fit). I do remember a few years ago that there was an AK-based air rifle they banned because they felt that it was 'way too feasible to strip it down and turn it back into an AK receiver.
