Lots of folks make 5.56 uppers.  Who makes the big boys in bbl lengths under a foot?  I'm hoping specifically for something that still uses USGI mags, and preferably something I can pin onto my Carbon-15,

Wish I could see the BOHICA site....

Teppo Jutsu makes them in .458 SOCOM in any feasible length you want.  I know he will do down to 10 inches and mine is 12.5 inches.  Sadly, we ARE still taking about barrel lengths.

yike's that sounds like a hand full.

NOOOOOOO!  

I just found out my original Pro Ord Carbon 15 pistol lower only takes Pro Ord uppers.  Bushmaster is telling me they had different takedown pin positions.  WTFO?  That would've been usefull info before I bought it......

I guess I'm in the market for a whole pistol now.  Anybody want to build me one?  I have to confess I'm scared to try it.  I've field stripped M-16 and M4s since I was 18, but I've never built an AR.  Guess I could get afull lower and a full upper and stick em together. ...  Why do I think I'm about to spend a butload of cash?

Haha I'm no gunsmith but I've assembled quite a few and if you can field strip, you can build it.  It's not that hard and there is really nothing to be afraid of.  The barrel needs to be torqued and that's about the hardest part.  The little roll pins are a piece of cake and despite a few people loosing springs it's actually quite simple to assemble.  Go ahead and try it yourself!  Trust me you'll be proud when its finished.  Except you'll soon hate me for suggesting you DIY because you can't just do one..  'This is madness!!!'  ... 'No... THIS. IS.  SPARTA!!!    B.R.D.!!!!!'

Not too many people, unfortunately.  If you don't order from Marty (@ T-J), your next best bet would be to order a big bore bbl from someone (anyone) and send it to ADCO to have cut down.
ETA: Here are two threads I thought you might want to take a look at:





Variant Calibers For Home Defense..... Out Of Short Barrels?  (my archived Apr 2008 thread in the "Variant Calibers" forum)





AR pistol uppers- sources & descriptions (a tacked thread here in the "Pistols" forum)
ETA2: Bah! Just realized you're not a Team Member... most of the relevant info from that thread can be found in my posts in the second thread.