Can the bolt on a Browning 71 be milled to accept the 98 bolt peep?

The bolts on the Browning's are supposed to be a lot harder than the originals and harder to machine.

I believe there was a reproduction bolt mounted peep made that was redesigned to minimize machining of the Browning bolts. Not sure they are still available.

Does the process of milling the slots in these bolts damage them in any way? Do they need to be heat treated or anealed or anything? Or is it done slowly enough with cooling oil that nothing else is necessary?

I never had one altered, so can't help. I just remember reading that somewhere. You'd need to talk to smiths with specific experience in machining them.

If you're looking for something that's really rugged, the XS peep sight seems like a good option. I have some vintage steel Redfield and Lyman side mounted aperture sights fitted to mine and happy with them.

The other point I'd like to make is that the vintage sights I bought didn't fit the Browning 71 or 1886 perfectly. I had to have a little metal removed for a proper fit. Either the holes in the receiver for the peep sight are a little lower than the originals or the profile of the top of the receiver is ever so slightly different.