[Sharkbite]

1. Can I register one of the rifle lowers that I already have as an SBR or should I start with a virgin lower?

Yes you can use an existing lower. All 4 of my SBR's arr built on guns i already owned

2. Can I freely swap different (short) uppers onto one registered lower? yep again. I have a 223 and 2 different 300blk uppers that i swap around on 1 of those lowers,

3. Would I be breaking any rules to put a full length upper on a registered SBR lower? (Not that I can think of a reason why I would.)

. Nope. Swaping a standard upper does not remove the lower from the NFA registry. You can put a full size upper on the lower and travel to nonNFA states and across state lines without doing a 5320.20 as well