I was looking at the Para TTR specs last night and thought it might be possable to convert a standard AR type rifle to the Para's recoil system and side folding stock. From what I understand a gas operated with a op rod attached to the carrier which slides overtop of the short gas tube. Surrounding the op rod is the recoil spring that stands in for the conventional buffer/spring system. Also the carrier has been shortend 2.9" so that a folding stock can be used. Watch this vid at about the 1 min mark you can see just how it all works. http://www.para-usa.com/new/video_ttr.php
The Para arms setup reminds me of an improved Olympic arms piston system. A piston driven upper is on the "to do list" for me at some point. I was thinking of an off center gas piston to keep the heigth low enough to use standard handguards or free float tubes if you wanted.