The finer pitch 6-48/8-40 special scope and sight mount screws, put more threads in a thin section than the standard fine pitch screws...
Finer pitch also has a slightly thicker cross section, which helps in shear.. Which scope mounts are exposed to during recoil..
If receiver was so hard you had to drill it with carbide, you would not be tapping holes.. Spot annealing required.
Really if you have never done this, send it to a competent local gunsmith. Nothing like drilling a scope mount screw hole, right through upper bolt locking lug shoulder...
Buy the time you buy the special taps (both plug and bottoming), you are approaching what it costs to have it done..
OSG web page flakey, did not see any 6-48 or 8-40 UNEF taps...