Looking to build or buy my first SBR. Looking at getting a Seekins or Noveske and having it engraved. Feel free to agree or disagree here. Looking for informed opinions on any of this. I have two cans that are waiting on paper work. TBAC 30-P1 and a Sparrow 22. Here is where I am uncertain. I want the SBR mainly for a 300 blackout setup shooting mostly if not all suppressed both sub- and supersonic. BUT I would also like to have the option to run a 5.56 shorty upper. From what I understand your SBR lower has to be marked with a specific caliber. Now am I going to get myself in deep chit by putting a 5.56 upper on a 300 marked SBR lower? Same deal if I got a notion to throw a 22 upper on there with the Sparrow. Thoughts?