Why not call the ATFE office and ask? What Jfightingfalcon is giving you is Mich State law, you are in AZ. Mich State law has nothing to do with someone in another of the 50 states. When I call in a NICS background check, the serial number of the firearm is not asked for, although it is recorded later on the 4473 by the seller. One of the final questions asked during the NICS background check is, "Type of firearm". Meaning, is this a long gun, handgun, or "other' firearm. IF this is a new receiver, that has never been a rifle OR a handgun, it can be used to build either one. IF it was a rifle when it left the Savage factory a 'variance' needs to be granted from ATFE before it can be assembled as a handgun. The NICS background check that you fill out when you buy from a dealer is a background check on YOU, not the firearm. The reason they question is ask about "Type of firearm", is the age of the buyer. As the text you referenced, you must be 21 to buy a handgun or a 'other firearm" (a receiver) because a 'other firearm' can be assembled as either a long gun or a handgun. As the legal owner of the "other firearm", technically, you have become the 'manufacturer'. That 'other firearm', turned over to a gunsmith for assembly is preforming a 'gunsmithing task', not manufacturing a firearm as he doesn't own the receiver. That is the way I understand it, but, if I was to be approached by client to build a handgun from a receiver owned by the client, I would call ATFE and ASK!