In an effort to focus on different areas of the AR pistol...

.223 AR pistol - uppers

What?  No replies!?

Okay..... me shall start.
Barrel lengths
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.  

Hybrid
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.

Oly Arms
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.