Can I have a 16" 5.56 AR, and a registered 9" 300blk AR that I put together? If yes, then what about additional uppers for the SBR lower? They would be longer than the original 9" barrel but possibly other calibers ( i.e. 68spc). I've read that there's no reason to list additional uppers as long as I can return the rifle to it's original configuration and as long as the additional uppers are longer than the registered length, 11.5" 6.8spc for instance. Basically I was planning to use my "Cal-5.56/.223" receiver only for the 16" 223 and use a Multi cal receiver for the SBR.