Might try the shim on the bottom of the mag spring next to the floor plate.
Have you tried another spring in the Mauser?
I have not tried another Mauser spring yet as I don't use that rifle very often. The M70, though, sees the field at least once per week. How would you suggested crafting the shim? A wooden wedge? A steel wedge? Toothpicks or coins? (don't laugh, I've seen worse). I'm curious as this may be the simplist solution until I contact Numrich Arms for magazine spring replacements.
Thanks
RM