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.