I got my Form 1 for my SBR a few months ago and have been building another upper for it (which I have a pistol lower for as well...which will also eventually have a form 1 sent in for). In the mean time, I would like to put my new upper on the SBR lower. Do I have to report to the ATF that I will be using the lower in a different configuration with a different barrel length? If so, is there a form I have to fill out or how do I do it? I thought I was good to go as long as I kept the upper for the orginal config, but I was informed by a local gun store and associate that I trust that I really should report to the ATF. Is this just being extra cautious, or is it required?
Thanks.