You can order and recieve any lenght upper that you want right now and have it shipped to your door.

Just make sure you dont have a lower reciver (rifle) in the same house or your possesion.

That is classified as "Constructive intent".

I wouldnt take the chance if you have a lower other than your pistol lower.

If your stamp is in the mail and almost her, Im sure you could justify it, but would it be worth it.