yeah, its just that the ATF can be dicks, so if they see a non-sbr lower with no upper, and you have short barreled uppers stored in the same place, they probably could do something, even if you have an SBR lower. But more what I was wondering about was whether the ATF cares or has any sort of legal jurisdiction on you changing the caliber of the upper to something that doesn't match what the SBR was originally registered as, and whether it matters if you keep the upper that was the original caliber. So say you register as 5.56, then decide you want it to be a 300 blackout instead and build another upper, then sell the 5.56 upper.