Here's my setup, it started as an Airsoft GI G4-A3 (essentially, a custom G&G CM):
The scope is only mounted half way because those scope rings are garbage, I have better ones on order. Also, that attachment in the side of the RIS is my camera mount....
I have got this gun firing 400fps (on the dot) at 25rps.
The only internal work that may be in for the future is an AWS Raptor mosfet (when they are out) and, possibly, a high speed helical gear set (not too high though).