Re: who make a pistol AR lower.....
From what I've heard it's okay to have both pistols and rifles, but you just better not have a pistol upper and rifle lower without also having a pistol lower.
I think the way they justify that is if you didn't have the pistol lower, the only possible use to you of the short upper is the illegal assembly of an SBR; whether you do it or not.
All of this is of course all those fancy four and five letter acronyms YMMV, IANAL, etc.