It seems to me that there are two types of AR pistol carbines. One that's purposefully built to shoot pistol rounds and two, an adaption kit to apply to an AR 15. Yes? If no, please pass on some knowledge. And, What's the good and the bad with either design? I saw a Spikes lower for 9mm that took Colt mags. I've never seen many extended mags for colts and it seems Glock has an edge in this area. The lower $265 which seemed like a pretty good price. Thoughts?