While I don't know what state you are in so I cannot speak for your state laws. When a stripped lower is Acquired by a dealer and then sold it is done in the category of "other" and "receiver". The feds and all paperwork associated do not consider it a rifle or pistol.
"Regardless of whether justified of not, you will feel sad about killing another human being. It is better to be sad than to be room temperature." - Joe B Fricks.
01 FFL, GOFFA, NRA, GOA, IAVA, VFW, ABATE of Iowa D15.