Anyone who has handled the SRC version of the MP40 will tell you the magazines wobble like a drunken Bambi on ice skates. Now there's several different solutions for this on the web (Google is your friend!), but a really easy fix I have discovered is to simply put some PVC Electrical insulation tape on the front and back faces of the mag.
This has reduced the wobble to almost nothing, they go slot in with a nice click and stay put. I've also found that with some mags you need to add a second layer of tape to stop the wobble, it varies from mag to mag, you'll just have to add some tape, try it and then add some more if you need to (it will also depend on the thickness of the tape you use) but of course be careful not to add so much they become difficult to force in or remove.
I also tried putting the tape in the mag housing itself but it always seems to come unstuck after a few mag changes 
I've yet to have any trouble with the tape on the mags.
Hopefully this will help someone. 
I've fired a bullet on every continent. Nearly hit someone, too.
