I've own the Full alluminium SRC genIII hk416 (yes it really has a aluminium body :P) for about a year and a half now, and this gun is a beast!!
Really nice internals, the M120 feels more like a m110 on mine, (360 fps), and the piston has a 100% compression/airseal, wich means if you put your finger on the air nozzle, the motor wouldnt be able to pull the piston back.... :P
heres a review on Airsoft pacific with pictures of the internals on SRC's gen 3 gearbox.....
http://www.airsoftpacific.com/content.php?page=51