I still say they're barking up the wrong tree with the AR-15.
The original system was designed around aluminum. Building the same exact design to the same dimensional specifications out of plastic will have predictable results. Change the specifications and you have a new design; "not quite" an AR-15, isn't.
Now, build something up around something
which was originally designed around polymer, and you'll have something.
There are quite a few firearms build around polymer. FS2000, PS90, Steyr Aug, etc.
Glocks and other polymer framed pistols would be an interesting experiment but they have substantial metal reinforcement at key parts of the frame (to keep the slide from violently flying off towards your nose...). To do those properly you need a pretty sophisticated mold to hold precisely crafted steel reinforcements at precise points (introducing intersting flow issues in the mold, I'm sure). On top of that.. you some REALLY high strength polymer, not cheap plastic.
So the relevant question is, what's the strongest polymer material that you can buy, and print with?
And is there an existing design which will work at those strengths? If not, what reinforcements or alterations are necessary on the stress points to MAKE the design work?