3-5 mil wide and no more than 4 mil deep. Its for my knife making and the steel is hard all the way through.Depends how big and how deep you want to go. I used to do a lot of scope mounts, occasionally a hardened receiver or slide would cause problems. I bought straight flute tungsten bits from Brownells (expensive) and away I went. Another way is to spot soften the area to be drilled using a fine tip oxy torch and lots of heat sink where you don't want the heat to go. Is the area case hardened or tough all the way?
As above, also note if you get high carbon steel too hot while drilling you will spot harden it.Any decent make HSS drill should do the job, the key is to keep your RPM low, the commonest killer of drills in the workshop is to much RPM.