I have a JG G36K up for sale. It has the M4 mag adapter. The gearbox has been gone through, installed metal 7mm bushings, shimmed, greased, aluminum air nozzle with o-ring installed, teflon taped the cylinder head, and adjusted AoE. It is shooting pretty consistent at 325fps, so it's good for any CQB field or outdoor. It also comes with one high cap. Looking to get $100obo for it.
I have another spring that I could either give to you or install for no charge if you want a bit higher fps. I'm guessing the other spring is around an M120, but not exactly sure.