Stainless work hardens like mad. You want to use low speed and lots of feed pressure. Good quality cutting oil helps a lot.
Cheap, high RPM drills are a sure ticket to failure. I would recommend something under 25 FPM with a HSS bit in stainless.
Ed
I haven't put level marks on a 5 gal. yet, but I did this on a 6 gal. better bottle branded carboy using a digital scale for accuracy (8.345 lb/gal). 6 gal was pretty much right up to the bottom of the neck. Same goes for a 3 gal.
Ed