I garuntee there is already a thread on this. I could not think about keywords to get me to a post if it exsists.. I recently put together a CMMG lower reciever that I would like to register as a SBR in the near future. When I file my form 1 and am waiting for my tax stamp to come in, can I still throw a 16 inch upper onto the completed lower and shoot it as a rifle? I threw in a good trigger and want to add some more upgraded parts into it and would hate to have it sit in my safe for 6-12 months until approval