I've been hand drilling the countersinks in my cowl using some carbide-burr countersink bits with great results. Go slow, and check often. All have come out just right.
N430WB RV-7 #74152, tip-up. Final assembly.
Log at: http://rv7.wbencze.com
Defeating gravity one rivet at a time
VAF 2020 donation happily made