Haven't seen this here so here it is. The AR-15 lower does not qualify as a firearm. Very interesting read. I would buy a few extra uppers as they may someday be married to a lower meaning you have to buy a lower to get a upper. Also would mean you might not be able to have 10 different uppers for one lower.
https://www.thetruthaboutguns.com/the-atfs-definition-of-an-ar-15-lower-as-a-firearm-is-in-serious-trouble/
https://www.thetruthaboutguns.com/the-atfs-definition-of-an-ar-15-lower-as-a-firearm-is-in-serious-trouble/