CDNN has got the AA piston kit on sale for $129 in mid length for .750 barrels.
i am very tempted to try one since i haven't done a piston drive ar-15 yet.
only thing is i got plenty of other things to buy/build--
might be worth a shot before plunking down mucho $$$ for one of the other piston driven solutions like a SCAR or sig.
this sale expires at 16:00 central time.