But EP Armory claims that they start with the plug, then mold the rest of the lower around the plug, so a lower with a cavity never existed. If you machined an aluminum lower then filled the void with wax, the lower with a void did exist at one time.
Semantics? Maybe. But the BAFTE did issue a letter confirming that the EP Armory lower was compliant with the 80% rule. Problem is that BAFTE changed their minds.
If you look at the link showing the completion of the EP lower, it is not a simple plug that is the correct shape of the void. There appear to be grooves and a complex shape to insure that the white plastic cannot be easily separated from the black plastic. The plastic is all one piece in the end, all welded to gether, and intermingled, but arranged so that the white plastic serves as a marker. It has to be machined away for the lower to work.