Think I used the second smallest of these countersink bits
http://www.amazon.com/IIT-25410-Counter-Sinks-5-Piece/dp/B001GMRD10/
The reviews mentioning that the bits are dull are accurate. Some force is required to use them.
A six flute countersink would be a better option.
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=6+flute+countersink