In an effort to focus on different areas of the AR pistol...
.223 AR pistol - uppers
What? No replies!?
Okay..... me shall start.
6.5, 7 (7.5*), 10, 11, 14
So far, that is the lengths I have see commonly advertised, IIRC.
Regular style uppers
Nearly all uppers for pistols are "cut down" carbine uppers. They require a buffer tube of some sort to fuction. The DPMS kitty cat, M1S and others fall in this catagory. All parts (except barrel) are identical to rifle/carbine uppers.
Barrel lengths around 7" seem to have function issues. Most issues were traced back to the buffer system (see other topics on buffers) or w/ gas tubes.
I call it a hybrid; the Bushmaster pistol upper. It is a "regular style upper", but the buffer and carrier are modified. The carrier is shorter and a special buffer assembly must be used. The use of this upper in a rifle would not work w/out changing the carrier to a normal unit. (This is only speculation as I have not seen one, only pics). So far, the Bushmaster is the only unit I know of that uses a non standard AR parts.
This upper from Olympic Arms of Olympia, WA has an integrated recoil system. A buffer tube is not required at all. Unfortunately, the barrel length is 6.5" for this upper. This would work on any lower.
Note: (Da disclaimer) The "use on a carbine/rifle" comment is for explaination only. By no way I am supporting use of a barrel length under 16" on a "rifle" w/out the proper, LEGAL requirements.