Today a friend of mine and I were talking guns in the garage, and we ended up on the topic of NFA firearms. He asked me a question that really made me scratch my head. Here goes; "If you have a RLL or RDIAS installed in your AR15 does that make the AR15 a 'machinegun', thus able to accept a shorty upper? Or is the 'machinegun' the RLL or RDIAS and not the AR15 and you can't have a shorty upper, because then it would be a SBR with a 'machinegun' inside? My inclination is to say that with the RLL or RDIAS, then you have a functioning 'machinegun' in the form of your AR15, thus it should be able to accept a shorty upper. Then, again, if you take the RLL or RDIAS out, you are in posession of an illegal SBR. Can anyone shed some light on this?