yes, but, lack of a properly fitted receiver (m-16 spec shelf, hole drilled, & flat faced wall without block on receiver, etc), is proof that not all the necessary parts are in possessing to complete this build. You could buy an m16 BCG, m16 trigger group, FA sear, m16 height lower, even if you could find one flat faced, and buy every thing else, but if literally ONE aspect of that gun is lacking required to make it fire consecutively, it is not illegal, and you are not in violation of any ATF laws (unless ofc your local laws denote you cant have an AR in general lol). This is a common misconception because most people have never seen a FA AR-15 style, so they don't understand all the little complexities that make them diff from the commercial ones. Anyways, a more simple way to look at is this: if the ATF shows up to you residence, and, with all the parts you have in your possession, are not able to make your gun fire, the exact way you have them, in your possession, you are not in possession of an illegal gun. IF you did have all the parts required, down to every last detail, but unassembled, the "constructive possession" rule would apply. Because, yes, in that case, the ATF would be able to put your gun together and make it fire more than one bullet per trigger press = you are in possession of a machine gun.