I've got a stripped lower and upper that I would like to make into a short barreled rifle chambered in 6.8. I've heard Form 4 and Form 1 tossed around, but don't have experience with a SBR. I did the Form 4 for my suppressor, but would I be considered the "builder" for a short barreled rifle if I start with just a stripped lower/upper which would require a Form 1, right? It's an Aero Precision upper/lower stamped as "Multi-caliber". What do I need to do besides the photos and fingerprints? Which form do I submit?