I have my stamp for a 8.2" 300BLK SBR. I'd like to have a second SBR upper in the 8-10" range in 5.56, for use on the same lower. I think I'm allowed to put the 5.56 upper on my SBR lower provided I always have the original "stamped" SBR upper (300BLK) on hand right? I can be at the range with 5.56 upper and have the 300BLK upper at home? Also, any issue with keeping the second SBR upper (5.56) in the safe with other completely built ARs? No partially built lowers or lowers without dedicated uppers. Thanks for your help.