If I register an AR lower as an SBR am I limited to only one short barreled upper? Can I have two (or more) uppers that would be used on that lower? Does it matter if they are the same lengths? Different lengths? If I can have more than one to go with a single lower, then does everything change if I own more than one lower? I currently have two lowers and 3 uppers (a 14.5 with pinned muzzle brake (total length >16"), 16" M4, 20" govt profile). The 14.5 is going to be a dedicated .22lr as soon as my CMMG conversion kit gets here, and I'm looking to add a 6.8SPC upper to the collection soon. If I register one of those lowers as an SBR, can two of these uppers have <16" barrels or will I run into problems with 'constructive possession' because the SBR upper could be assembled onto the other lower. Any advice would be welcome.