The first one talks about the legality of purchasing one after Oct 1st. Which will soon be possible once Engage collects the part and sets a price for their approved E4 pistol in 5.56/300 Blackout. Im sure they will announce it here in the forums once they are ready.
The second one is a helpful one as well in determining what/how an ar pistol can be built.
I just checked another source if we have a Virgin lower either from a 80% or a legally transfered before oct 1st we can build an AR Pistol but just like the rifles we can not sell them!