It is illegal to sell guns with certain characteristics (e.g. fully automatic fire, select fire) without possessing a Federal Firearms License which carries cer

It is illegal to sell guns with certain characteristics (e.g. fully automatic fire, select fire) without possessing a Federal Firearms License which carries certain onerous requirements of the seller. It is not however illegal to own or make such guns. Because the AR15 is so modular in nature and parts like the stock and barrel don't dictate if the firearm is automatic, federal laws dictate that the lower receiver is what is actually considered the firearm. Since regulated and unregulated AR15 lowers are mostly identical, and the point at which they diverge is a nonfunctional state, manufacturers can make incomplete or 80% complete AR15 lowers that aren't considered firearms. You can then take these lowers and complete them yourself at home to have an unregistered firearm that you could not otherwise posses without registration.