I quickly learned to look at the 1st Gen DPMS LR-308s and AP4s as starting points to work off of, using the receivers and BCG as a donor action, then replacing most everything else.
I see Aero M5s and PSAs in the same light, but since there are enough working rifles to just grab, glass, and go shoot, there’s no need to waste your time if you really want a large frame for some reason.
I personally am done with large frames, which is sad to admit, being such a big AR-10 guy for so many decades. I never thought we were going to have access to them, given the trajectory of anti-2A infringements being passed in the late 1980s and the 1994 AWB.
KAC came out of their normal areas of expertise with suppressors in the 1980s, and made the SR-25 with Stoner, which changed everything.