Mausers usually are not very hard. The answer may be either spot annealing (which can be done with o-a torch set to a tiny flame or even an electric solder gun) or the use of a carbide bit. But if you use the carbide bit, you will also have to use a carbide tap and they break very easily.
I suggest spot annealing.
Mausers usually run around 35 RC or less, not hard compared with U.S. GI rifles that run 59+. I seem to recall a note somewhere to the effect that if a Mauser is very hard throughout (they are normally surface hardened) it might have been heated and quenched and be brittle. This may be faulty memory, but checking might be advisable.
Jim
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