I have a lower in need of an upper and have been thinking about a 18"OR 20" flat top to scope and shoot targets to 200yds.  
Wile in the store I saw a armalite 24" gun with a fiberglass tube and stainless barrel. I dont know what chamber they use but it is an 8 twist and looked great.
 
 Why couldnt I cut this to length and have a great medium range and handy shooter?

much for the upper?

After paying Armalite for premium barrel, then gunsmith to chop and recrown, I bet you can buy a custom barrel for the same money from Frank White at Compass Lake. Down in your state, too. Wait until after after camp perry, I bet they are busy as a one armed  paper hanger in a butt kicking contest .

I tell you what, a CL 18" barrel will resell for a buttload more than a chopped off Armalite!

Yup, for the same money you could just do it right the first time.

You could just go with the 16"...at 200 yards, you're not losing anything to speak of (a few fps, but nothing that's gonna hurt you that bad at that range).  Armalite uppers aren't cheap...$550 for the (T) models, with the float tube and flat top.  You could get a Rock River, or an upper from J&T or similar place for $100 less.  

Go to Model 1 and check out the CAR upper halves...you could do a 16", float tube, Stainless fluted heavy barrel, 1:9 target crown, with b/c for $445.  This wouldn't be a bad upper for what you want.  

Compass Lake does nice work...you should call them...they're prices are kinda hard to figure out on the website, but you may be able to get what you want there.  

http://www.wilsoncombat.com/store.cgi?&shop=city&Category=30400

The wilson is an 18" and looks nice. Upper alone is 699.