Drilling/tapping color case hardened tang

A friend has asked me to install a Marbles tang sight on his Rossi Mod. 92. After setting it up in the mill I find the tang is too hard to drill with conventional drills Carbide seems to be in order. I contacted Brownell's tech dept for some advice and they suggested grinding down through the case hardening (about 6-7 thou)and starting the drilling from there. Problem is once I get through the case hardening, how is the steel below? Will a new conventional tap (10-32)suffice or should I try to find a carbide tap? I don't want to botch the job, so if someone has done what I'm attempting and will follow up I would appreciate it.
Thanks

It should be soft underneath as a case hardening done on any type of carbon steel would likely result in a part too brittle for service.

I saw a kink once where the recommendation was to spin a piece of drill rod against the hard spot until it was smoking. This was supposed to anneal a spot large enough to start a drill. However, I have not tried it.

you can spot anneal the area where you plan on drilling with an AC welder and a small piece of carbon (such as from a D or C size battery)

grind the carbon to a small point on one end and then place it in the positive lead of the welder, next attach the ground cable to the part in a place where it would be unseen if it happens to arc(chances are it will not arc if pressure is held)

with the welder set to it's lowest amp setting and turned Off, place the carbon rod in the exact location where you will be drilling and make contact there by holding it with downward force.
next turn the welder on in short bursts by flipping the switch on and off with power being supplied only about 10 to 15 seconds between bursts.

this will make the metal under the carbon contact get hot and soften in that spot and not bother the surrounding area.
afterwards it can be easily drilled with conventional drill bits and easily tapped as well

i would suggest trying this on a scrap piece beforehand to get the feel of how much heat is needed to soften harder metals, then proceed to the gun part

i have used this for many years to drill and tap hard receivers

Get a hard steel, straight flute drill. They work great. Use a new taper tap with care as the skin next to hole is hard on top and bottom. Sometimes you can touch corner edges with small pointed grinder to remove hardness next to hole (bevel edges). Use a drill bushing setup if you have such, and keep tap straight.