You are right, it's not really square, but the rounded edges can be chiseled away fairly easy, especially if the drill bit has the chisel built right into it as

You are right, it's not really square, but the rounded edges can be chiseled away fairly easy, especially if the drill bit has the chisel built right into it as jacquesm's earlier image showed (http://www.machinemart.co.uk/images/library/range/large/0104.jpg http://www.machinemart.co.uk/images/library/range/large/0104...). although if it's hard wood, you may need to use a solid chisel and a hammer to pound the heck out if it.