I have used a " Unibit" (cone shaped multi size) drill bit for my NMO mounts. The one I prefer maxes out at 3/4 inch. It doesn't need a pilot hole, or "walk" on the metal. The only thing I do is USUALLY start the hole from the outside and finish it from the inside. Then there are no burrs, and I don't have to file anything.
Don't get me started about how I tap into the battery cables.
.... I am an electrician by trade.
de Howard, N9KTW