•A factory fresh AR lower receiver that has never been part of a firearm can be used to build a pistol, carbine or rifle. If a lower receiver is built into and registered as a pistol first, it can be stripped down and converted into a rifle in the future. If the receiver built into a carbine or rifle first, it must always remain part of a rifle and cannot ever be used to build a pistol.
From American Rifleman July 14, 2016:
https://www.americanrifleman.org/articl ... s-at-home/
Just my understanding.