Silicon Graybeard’s already done a great job on the “leak” of ATF’s proposed rule-making on 80% receivers.
All I can add is that, as so many are available are made of injection molded polymer, and are therefore quite inexpensive, it might not be the worst idea you might have to buy several of various sorts and download both the manuals and videos from the vendor’s web site.
Just sayin’.
Make yer own:
https://ia600201.us.archive.org/33/items/AR-15_Scratch-Built_Receiver_Blueprint/AR-15_Scratch-Built_Receiver_Blueprint.pdf
Figure a good weekend to make one. lyout on Friday night, cutting and drilling on Saturday, and assembly on Sunday.
Just sayin’