Without the somewhat pricey tap, those oversized 6-48s have been less than stellar, and can CREATE problems, as opposed to solving em' .
At this point I'd be taking a very long look at 8-40, for the most reliable and budget friendly solution.
Drill to # 28, and use a carbon steel tap, as opposed to HSS,...if things go south, easier to remediate. If the tap's in the least hard to start, go to a #29 hole,....understand that doing this PROPERLY calls out having a 3 tap set,....but MANY fine repairs have been achieved with but one. Don't sweat the counterbore size in your base(es),......spin the head of the 8-40 down in a drill chuck, cold blue it, and get back out in the field.
Compliments on your concern about reliable zero.
GTC
