SR-556 Ruger piston driven troy addition. not gas. they dont make these anymore and harder to fing. i put a geissele combat trigger, Burris AR-332 scope has troy flip up sight, handguard and magpul stock and trigger guard. may be 3 mags through it. i bought it new. great shooting gun.$1,500.00 OBO scope and trigger worth $500.00 469 four five zero 5390 David