This came from a cyma platinum mp5 (not the mp5k) which had a mosfet issue. It had a quick change spring. From memory the compression is good but it's been a while since I've had it open. There's no mosfet or motor or selector plate. But if you want to stick a perun in it I'm sure it'll be fine. 

If you want me to open it up and send photos I'm happy to do that. I'm just not sure how much interest there'll be. 

£35 posted.

Patrolbase sold these for £130ish but that included the mosfet and motor etc.