Hey, what a coincidence as I'm putting together a CA B&T MP5 with a Systema S170 Magnum set. Of course I swaped out the 550fps spring for a 400 one.
So far the mech fits the body shell and hop-up pretty nice tight, the grip is a little off on the allignment but it's just a hair off over the motor hole but should be no issue from what I can tell so far.
I'm just waiting on some remaining parts to arrive and then I can assemble the entire gun. Yes, Skru is right, you'll need to use a large battery so the full stock is about the only good solution. A 9.6v NiCAD or higher is recommended
Feel free to PM me in a week or 2 for results
Cheers, Steve