The 80% AR-15 Lower Receiver started it all, this was the first commercially available receiver on the market and had a determination from the ATF saying it was not a firearm. Billet 80% lowers begin their lives as solid blocks of aluminum that are CNC machined from beginning to end, https://deanekpuz.iamthewiki.com/7618769/eine_unvoreingenommene_sicht_auf_80_ar_lower