The lower reciever has to be a pistol lower first.
It can be a pistol lower, built as a pistol, then built as a rifle and go back and forth...
Or a stripped lower (never built as a rifle) first as a pistol, and can then go back and forth.
A lower built first as a rifle can never be a pistol.
Dont ever put your <16” upper on a rifle lower unless registered as an sbr.

