Re: mosin nagant-91-30 customizing questions
In talking with my son-in-law, who had his good gunsmith modify several of his M-N to take period mounts, I was surprised to find that even this experienced gunsmith broke a tap in a M-N receiver. He had to send it out to have the broken tap removed by EDM (Electrical Discharge Machining).
Just as the appearance of M-N is a bit crude, I suspect the materials they chose to make them out of may also have been crude, as well as the heat treatments. I suspect hard spots internally that may impact the experienced metal workers, as well.
The bottom line is don't drill and tap the M-N receivers yourself. Give that responsibility to a professional who has to eat the added cost of broken taps or drills.