I'm trying to think what would cause this with my CETME, never had that problem. Is this something that just started with a rifle you've had a while? Is it a sheetmetal or cast receiver? General condition of the rifle?
I'm no pro on this rifle maybe someone with more knowledge will chime in.
Couple of things, I'll asume your rifle is spotlessly clean and well oiled. It does this with all 4 or 5 mags you have tried in it. You have checked the headspace and found a problem. You get this with several types of surplus ammo.
Here is some CETME stuff you probably already know. Good Luck, Ken