Why is it you don't want to just buy a Carbon 15 pistol?
RRA and others make AR pistol uppers (and lowers, for that matter).
Model 1 sales, too: http://www.model1sales.com/index.cfm...ategory_id=332
The main legal issues are that it can't have a buttstock and the lower can't have been a rifle at any point in its life.
Most of the parts (LPK, BCG, upper receiver, pistol grip, upper/low receivers) are the same parts as AR-15 rifles. Most things that are not are just shorter versions of the same (barrel, gas tube, buffer tube, handguard...).