Don't know about the carbide end mill as it is not likely you'll find one in tap drill size. If the receiver is hard start with a carbide center drill and drill until the diameter of the hole just exceeds the diameter of the tap thus producing a small countersink. This will ensure that the drill doesn't have to deal with the hardened skin though a carbide drill may not be a bad idea. A spiral point plug tap and tapping fluid to start the threading process and if it is a blind hole a bottoming tap are the tools you'll need. A word about taps, there is a tap manufacturer by the name of Reiff and Nestor, they have been in business since 1908 and produce the highest quality taps available. They cost somewhat more than other manufacturer's taps but the advantage is this, R & N control the tooth to tooth spacing of their taps more closely than other manufacturers. The result is a tap that requires noticeably less effort to use, a critical factor when tapping #6/#8 holes in hardened gun steel. Try a side by side comparison in a scrap piece and you'll see how much easier the R & N tap cuts. Use a new tap for each job, the cost is well worth practically eliminating tap breakage. Good luck with your job.
I checked them once before and couldn't find taps in the 'standard' sizes that gunsmiths use; ala 6-48, 8-40 etc... Every one I looked at stopped at 6-40 and 8-32..
