I don't get it, polymer or metal an 80% receiver is an 80% receiver.
As other posters have attempted to explain, the ATF is claiming that the latest generation of these (EP Armory) polymer 80% lower receivers are being manufactured and
then having the FCG pocket filled. That would give the ATF jurisdiction, because they would be firearms with the FCG pocket filled - not 80% receivers.
However, I happen to have 2 of the latest generation of those receivers (the latest to ship, anyway). I can tell you, at least to the limits of my experience, that the ONLY indications in these lowers, is that the FCG pocket 'block' was molded first. Then, the rest of the receiver was formed around it. And the block that fills the FCG pocket isn't something that can be removed without milling it out. It has notches and protrusions all around it, that, after the receiver is molded around it, interlock with about 18-20 ribs in the receiver walls, 4 partially-formed holes where the trigger and hammer pin holes get drilled, the unfinished trigger hole, and notches where the selector holes get drilled. It's also a tougher polymer than the receiver, itself. So, attempting to pry the FCG pocket 'block' out will destroy the receiver.
The receivers in the middle of all of this:
(Sorry about the odd sizes. The forums got angry about file size and I had to do some non-standard cropping to minimize the "noise" in the photos.)
And... where mine are now - hanging from a nail, next to the unsafe black powder shotgun - because they're trash.
Every dimension on these things is wrong, before you even start removing material. (Don't waste your money, even if things do turn out okay for EP Armory; unless they fix the mold(s). They'll give you nothing but headaches.)
I've considered posting a full review and list of defects, incorrect dimensions, and improperly located drill guides; but it really doesn't matter. The bottom line is: Don't waste your money. Even if you mill everything perfectly, the rest of the receiver will cause problems.
I did get one of the receivers to run (for the most part
), despite the problems with the lower. But, a keen observer will notice that there is no selector/safety. That's because the receiver's overall height is too short (the deck is too low) and the selector hole drill guides are too low.....