I changed the receiver threads to 8x40 on a Rem 700 receiver over the weekend. This particular action was tough as hell to tap. I broke a tap in the number 3 hole which, had I not been thinking several steps ahead, could have been disastrous, but the hole was centered under my milling machines quill which made removing the broken piece a breeze. Yes, I can still break a tap with the best of 'em!
Hint, if you are going to do it yourself, don't be tempted to free hand it. Use at a minimum, a drill press, and center the hole under the quill, use a center in a chuck to guide the tap straight in. Or better yet, hire it done and let the gunsmith sweat it.