Quoted:
OK, it's been about 4~5 years since I have made a F1 SBR, and that was an AK.

Now, I'm thinking about registering 3 AR Lowers and I have some questions.

(1) If the lower is marker Caliber = Multi, I still need to designate a specific base caliber and note "Caliber = Multi" in the additional description block. Right? You have to designate the caliber you intend to build, and mark your gun with that caliber. The "multi" marking on your lower is irrelevant and should be ignored.

(2) As far as barrel length, could I just put 6.0" and be covered if I wanted to swap uppers with longer barrels? Again, put the length you intend to make.

(3) Ideally, I'd like to have 3 lowers and maybe 8 ~10 uppers that I swap on and off. Maybe different calibers too. They really don't make this too easy, do they. Any ideas on the best way to accomplish this? form 1 3 lowers and the buy 8-10 uppers and swap them on/off at will.