Very simple and easy to understand. If I were a noob (still am, sort of) I would choose this gun. Why? Cause the version 6 is a dream to work with. Easy to open and close and no need to re-seat the motor either like Ver 3 AK gearbox.
Oddly enough the Ver 2 Spring guide can be used just the Tappet and Cylinder head are special to their own make. After working with Ver 2 and Ver 3 mechboxes and totally destryoing them and rebuilding them and destroying them again I learned the ways of the mechbox with the help of crunchmeister, m102404, CDN_stalker and more.
If I knew about Version 6 gearboxes in the first place I would have got the P90 before a MP5.
Only downfall is that when I first opened the gearbox the tappet plate snapped somehow, but after replacing it with parts I got from airsoftparts.ca it was working great and never snapped