This beauty will allow you to externally adjust your hop up. The install does require you to drill and ideally tap a hole in your upper reciever.
For instructions, please go to the following link: http://www.airsoftsniperforum.com/43-longrange-aegs/25250-easy-precise-tdc-hop-up-adjustment-m4.html#post225762
Besides this kit you require 1 short m3 bolt, a long m3 bolt with a hexagon head and a m3 ball plunger.
Once installed you will be able to finely adjust the hop up as there are 36 positions per rotation of the bolt!
This will allow you to very precisly fine tune your hop and as such get maximum range and accuracy.