The king arms hop up
http://www.airsoftparts.ca/store2/in...roducts_id=514
The barrel clip on this is a bit of a bitch to get on. Once it is on though it holds really damn tight. The orings also help with airseal. I noticed a 30fps difference from stock once it was installed 320fps-350fps.
Tip. Their adjustment knob screw is kinda shitty. keep your old one around just incase lol.