Yes, it is common to find hairline cracks in the magazines. I'll attach a more extreme example from a gun I got from Simpson, Ltd-it had been stuffed into the styrofoam spare mag slot so I did not know until I removed it. I would also caution buyers to look out for grip screw distortion which is ugly and may indicate the grips were shot loose. I have not fired any of my Mauser Parabellums, none of mine seem to have been fired since the factory tests.
