From Reason:

“ATF has maintained and continues to maintain that a partially complete frame or receiver alone is not a frame or receiver if it still requires performance of certain machining operations (e.g., milling out the fire control cavity of an AR-15 billet or blank, or indexing for that operation) because it may not readily be completed to house or hold the applicable fire control components,” the document says. 

ATF adds that new restrictions would apply if the partial frame or receiver is indexed or dimpled to indicate where to drill, or through “the aggregation of a template or jig with a partially complete frame or receiver.” Minus such clear markings or accessories, though, unfinished receivers would appear to retain a viable market without having to go to the raw blocks of aluminum and polymer necessarily exempted in the rules (unless you want to subject hardware stores to gun regulations) and with which Ghost Gunner has a distinct advantage. Wilson still sees an opening, though, in states that have tighter rules than those imposed by the ATF.