Roni has a new member of their conversion kit line that debuted at the 2011 NRA Convention. This unit has 16 inch replacement barrel for your Glock 21 in addition to the drop in carbine body. This allows the reliability of Glock to be exploited in a .45acp carbine.
One of the major advantages to this kit is that it does not require an SBR.
Roni's Glock 21 Based Carbine
I really like the ergonomics of this ugly thing. It is light weight, comfortable and adjustable, has 30 round mags available, plus it has plenty of rails for accessories.
--I would have bought one on the spot if possible. $299 for the kit!